Encoder, a decoder and corresponding methods of chroma qantization control

ABSTRACT

A method of obtaining quantization parameter (QP) for chrominance components based on QP for luminance component, wherein the method is performed by a decoder or encoder, and wherein the method comprises obtaining a luminance QP for a luma component of a coding unit; obtaining a QP index (QPi) based at least in a part on the luminance QP; and obtaining a chrominance QP (QPc) for a chroma component of the coding unit based on the QP index by using a preset lookup table, wherein the maximum difference between the QP index and the chrominance QP is equal to 3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/RU2020/050131, filed on Jun. 22, 2020, which claims priority to International Application No. PCT/RU2019/000445, filed on Jun. 21, 2019. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

Embodiments of the present disclosure generally relate to the field of image and/or video decoding and more particularly to apparatus and method for quantization control.

BACKGROUND

Video coding (video encoding and decoding) is used in a wide range of digital video applications, for example broadcast digital TV, video transmission over internet and mobile networks, real-time conversational applications such as video chat, video conferencing, DVD and Blu-ray discs, video content acquisition and editing systems, and camcorders of security applications.

The amount of video data needed to depict even a relatively short video can be substantial, which may result in difficulties when the data is to be streamed or otherwise communicated across a communications network with limited bandwidth capacity. Thus, video data is generally compressed before being communicated across modern day telecommunications networks. The size of a video could also be an issue when the video is stored on a storage device because memory resources may be limited. Video compression devices often use software and/or hardware at the source to code the video data prior to transmission or storage, thereby decreasing the quantity of data needed to represent digital video images. The compressed data is then received at the destination by a video decompression device that decodes the video data. With limited network resources and ever increasing demands of higher video quality, improved compression and decompression techniques that improve compression ratio with little to no sacrifice in picture quality are desirable.

SUMMARY

Embodiments of the present application provide apparatuses and methods for encoding and decoding according to the independent claims.

The foregoing and other objects are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

The present disclosure provides a method of obtaining quantization parameter (QP) for chrominance components based on QP for luminance component, wherein the method is performed by a decoder or encoder, comprising:

-   -   obtaining a luminance QP for a luma component of a coding unit;     -   obtaining a QP index (QPi) based at least in a part on the         luminance QP;     -   obtaining a chrominance QP (QPc) for a chroma component of the         coding unit based on the QP index by using a preset lookup         table, wherein the maximum difference between the QP index and         the chrominance QP is equal to 3.

The technical problem underlying the method according to the present disclosure is that if strong quantization is applied, the chroma information may be completely quantized to zero, which would lead to the complete loss of color. Accordingly, the chroma quantizer step size is reduced for high QP values by applying a chroma QP mapping function/mapping table/lookup table. This method as described in the present disclosure improves the mapping table/lookup table with respect to providing less QP attenuation for chroma components in higher QP range.

In the method as described above, the preset lookup table may be defined by a monotonically increasing mapping function.

It is to be noted that a lookup table can also be considered as representing a mapping function, since in both cases specific QPi values are mapped to respective QPc values. Monotonically increasing means that with increasing QPi values the resulting QPc values are not decreasing, but are either increasing or are remaining constant for an increase of QPi.

The method as described above, may comprise adjusting the obtained chrominance QP by adding an offset value.

In the method as described above, the QP index may be clipped into a preset consecutive value range.

In the method as described above, the lookup table may be as follows:

QPi <35 35 36 37 38 39 40 41 42 43 >43 QP_(C) =QPi 34 35 36 37 37 38 39 40 40 =QPi − 3

In the method as described above, alternatively, the lookup table may be as follows:

QPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QPc =QPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 =QPi − 3

In the method as described above, the mapping function may be defined as

-   -   QPc=QPi for QPi<QPi1;     -   QPc=QPi−1 for QPi1≤QPi<QPi2;     -   QPc=QPi−2 for QPi2≤QPi<QPi3;     -   QPc=QPi−3 for QPi≥QPi3, wherein QPi1, QPi2 and QPi3 are integers         with QPi1<QPi2<QPi3.

In the method as described above, QPi1, QPi2 and QPi3 may the values QPi1=35, QPi2=39, QPi3=43.

In the method as described above, QPi1, QPi2 and QPi3 may have the values QPi1=30, QPi2=39, QPi3=43.

In the method as described above, the same lookup table may be applied for each chrominance component Cb and Cr or jointly coded Cb-Cr.

In the method as described above, different lookup tables may be applied for chrominance components Cb and Cr or jointly coded Cb-Cr.

The method as described above, may further comprise:

-   -   receiving a bitstream; and     -   parsing the bitstream to derive the luminance QP and syntax         elements specifying the lookup table.

The syntax elements, such as flags may indicate which particular lookup table is used.

The present disclosure further provides an encoder or a decoder comprising processing circuitry for carrying out the method as described above.

The present disclosure further provides a computer program product comprising a program code for performing the method according to the method as described above.

The present disclosure further provides a decoder or an encoder, comprising:

-   -   one or more processors; and     -   a non-transitory computer-readable storage medium coupled to the         processors and storing programming for execution by the         processors, wherein the programming, when executed by the         processors, configures the decoder to carry out the method as         described above.

The present disclosure further provides a decoder for obtaining a quantization parameter (QP) for a chrominance component based on a QP for a luminance component, comprising:

-   -   a first obtaining unit configured to obtain a luminance QP;     -   a second obtaining unit configured to obtain a QP index (QPi)         based at least in a part on the luminance QP;     -   a third obtaining unit configured to obtain a chrominance QP         (QPc) based on the QP index by using a preset lookup table,         wherein the maximum difference between the QP index and the         chrominance QP is equal to 3.

In the decoder as described above, the preset lookup table may be defined by a monotonically increasing mapping function.

The decoder as described above, may comprise an adjusting unit for adjusting the obtained chrominance QP by adding an offset value.

The decoder as described above, may comprise a clipping unit for clipping the QP index into a preset consecutive value range.

In the decoder as described above, the lookup table may be as follows:

QPi <35 35 36 37 38 39 40 41 42 43 >43 QP_(C) =QPi 34 35 36 37 37 38 39 40 40 =QPi − 3

In the decoder as described above, alternatively, the lookup table may be as follows:

QPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QPc =QPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 =QPi − 3

In the decoder as described above, the mapping function may defined as QPc=QPi for QPi<QPi1; QPc=QPi−1 for QPi1≤QPi<QPi2; QPc=QPi−2 for QPi2≤QPi<QPi3; QPc=QPi−3 for QPi≥QPi3, wherein QPi1, QPi2 and QPi3 are integers with QPi1<QPi2<QPi3.

In the decoder as described above, QPi1, QPi2 and QPi3 may the values QPi1=35, QPi2=39, QPi3=43.

In the decoder as described above, QPi1, QPi2 and QPi3 may the values QPi1=30, QPi2=39, QPi3=43.

In the decoder as described above, the same lookup table may be applied for each chrominance component Cb and Cr.

In the decoder as described above, different lookup tables may be applied for chrominance components Cb and Cr.

The decoder as described above, may further comprise:

-   -   a receiving unit configured to receive a bitstream; and     -   a parsing unit configured to parse the bitstream to derive the         luminance QP and syntax elements specifying the lookup table.

The present disclosure further provides a quantization method for quantizing a pixel information of a chroma component of a coding unit based on the quantization parameter obtained by the method as described above.

In the quantization method as described above, the pixel information may be a transformed residual of the chroma component of the coding unit, and the residual of the chroma component of the coding unit may be the difference between a prediction value of the chroma component of the coding unit and an original value of the chroma component of the coding unit.

The present disclosure further provides a de-quantization method for de-quantizing a quantized pixel information obtained by the quantization method as described above, based on the quantization parameter obtained by the method as described above.

The present disclosure further provides a de-quantization method for de-quantizing a quantized pixel information of a chroma component of a coding unit based on the quantization parameter obtained by the method as described above.

The present disclosure further provides a medium that stores bit-stream/or video sequence generated by a coding method that uses the quantization method as described above.

The present disclosure further provides a medium that stores video sequence generated by decoding method that uses the de-quantization method as described above.

Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention are described in more detail with reference to the attached figures and drawings, in which:

FIG. 1A is a block diagram showing an example of a video coding system configured to implement embodiments of the invention;

FIG. 1B is a block diagram showing another example of a video coding system configured to implement embodiments of the invention;

FIG. 2 is a block diagram showing an example of a video encoder configured to implement embodiments of the invention;

FIG. 3 is a block diagram showing an example structure of a video decoder configured to implement embodiments of the invention;

FIG. 4 is a block diagram illustrating an example of an encoding apparatus or a decoding apparatus;

FIG. 5 is a block diagram illustrating another example of an encoding apparatus or a decoding apparatus;

FIG. 6 is a block diagram illustrating the method according to the present disclosure;

FIG. 7 shows a lookup table, mapping QPi and QPc;

FIG. 8 shows another lookup table, mapping QPi and QPc;

FIG. 9 is a block diagram illustrating another example of a decoder and an encoder; and

FIG. 10 is a block diagram illustrating still another example of a decoder.

In the following identical reference signs refer to identical or at least functionally equivalent features if not explicitly specified otherwise.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the present invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

Video coding typically refers to the processing of a sequence of pictures, which form the video or video sequence. Instead of the term “picture” the term “frame” or “image” may be used as synonyms in the field of video coding. Video coding (or coding in general) comprises two parts video encoding and video decoding. Video encoding is performed at the source side, typically comprising processing (e.g. by compression) the original video pictures to reduce the amount of data required for representing the video pictures (for more efficient storage and/or transmission). Video decoding is performed at the destination side and typically comprises the inverse processing compared to the encoder to reconstruct the video pictures. Embodiments referring to “coding” of video pictures (or pictures in general) shall be understood to relate to “encoding” or “decoding” of video pictures or respective video sequences. The combination of the encoding part and the decoding part is also referred to as CODEC (Coding and Decoding).

In case of lossless video coding, the original video pictures can be reconstructed, i.e. the reconstructed video pictures have the same quality as the original video pictures (assuming no transmission loss or other data loss during storage or transmission). In case of lossy video coding, further compression, e.g. by quantization, is performed, to reduce the amount of data representing the video pictures, which cannot be completely reconstructed at the decoder, i.e. the quality of the reconstructed video pictures is lower or worse compared to the quality of the original video pictures.

Several video coding standards belong to the group of “lossy hybrid video codecs” (i.e. combine spatial and temporal prediction in the sample domain and 2D transform coding for applying quantization in the transform domain). Each picture of a video sequence is typically partitioned into a set of non-overlapping blocks and the coding is typically performed on a block level. In other words, at the encoder the video is typically processed, i.e. encoded, on a block (video block) level, e.g. by using spatial (intra picture) prediction and/or temporal (inter picture) prediction to generate a prediction block, subtracting the prediction block from the current block (block currently processed/to be processed) to obtain a residual block, transforming the residual block and quantizing the residual block in the transform domain to reduce the amount of data to be transmitted (compression), whereas at the decoder the inverse processing compared to the encoder is applied to the encoded or compressed block to reconstruct the current block for representation. Furthermore, the encoder duplicates the decoder processing loop such that both will generate identical predictions (e.g. intra- and inter predictions) and/or re-constructions for processing, i.e. coding, the subsequent blocks.

In the following embodiments of a video coding system 10, a video encoder 20 and a video decoder 30 are described based on FIGS. 1 to 3 .

FIG. 1A is a schematic block diagram illustrating an example coding system 10, e.g. a video coding system 10 (or short coding system 10) that may utilize techniques of this present application. Video encoder 20 (or short encoder 20) and video decoder 30 (or short decoder 30) of video coding system 10 represent examples of devices that may be configured to perform techniques in accordance with various examples described in the present application.

As shown in FIG. 1A, the coding system 10 comprises a source device 12 configured to provide encoded picture data 21 e.g. to a destination device 14 for decoding the encoded picture data 13.

The source device 12 comprises an encoder 20, and may additionally, i.e. optionally, comprise a picture source 16, a pre-processor (or pre-processing unit) 18, e.g. a picture pre-processor 18, and a communication interface or communication unit 22.

The picture source 16 may comprise or be any kind of picture capturing device, for example a camera for capturing a real-world picture, and/or any kind of a picture generating device, for example a computer-graphics processor for generating a computer animated picture, or any kind of other device for obtaining and/or providing a real-world picture, a computer generated picture (e.g. a screen content, a virtual reality (VR) picture) and/or any combination thereof (e.g. an augmented reality (AR) picture). The picture source may be any kind of memory or storage storing any of the aforementioned pictures.

In distinction to the pre-processor 18 and the processing performed by the pre-processing unit 18, the picture or picture data 17 may also be referred to as raw picture or raw picture data 17.

Pre-processor 18 is configured to receive the (raw) picture data 17 and to perform pre-processing on the picture data 17 to obtain a pre-processed picture 19 or pre-processed picture data 19. Pre-processing performed by the pre-processor 18 may, e.g., comprise trimming, color format conversion (e.g. from RGB to YCbCr), color correction, or de-noising. It can be understood that the pre-processing unit 18 may be optional component.

The video encoder 20 is configured to receive the pre-processed picture data 19 and provide encoded picture data 21 (further details will be described below, e.g., based on FIG. 2 ).

Communication interface 22 of the source device 12 may be configured to receive the encoded picture data 21 and to transmit the encoded picture data 21 (or any further processed version thereof) over communication channel 13 to another device, e.g. the destination device 14 or any other device, for storage or direct reconstruction.

The destination device 14 comprises a decoder 30 (e.g. a video decoder 30), and may additionally, i.e. optionally, comprise a communication interface or communication unit 28, a post-processor 32 (or post-processing unit 32) and a display device 34.

The communication interface 28 of the destination device 14 is configured receive the encoded picture data 21 (or any further processed version thereof), e.g. directly from the source device 12 or from any other source, e.g. a storage device, e.g. an encoded picture data storage device, and provide the encoded picture data 21 to the decoder 30.

The communication interface 22 and the communication interface 28 may be configured to transmit or receive the encoded picture data 21 or encoded data 13 via a direct communication link between the source device 12 and the destination device 14, e.g. a direct wired or wireless connection, or via any kind of network, e.g. a wired or wireless network or any combination thereof, or any kind of private and public network, or any kind of combination thereof.

The communication interface 22 may be, e.g., configured to package the encoded picture data 21 into an appropriate format, e.g. packets, and/or process the encoded picture data using any kind of transmission encoding or processing for transmission over a communication link or communication network.

The communication interface 28, forming the counterpart of the communication interface 22, may be, e.g., configured to receive the transmitted data and process the transmission data using any kind of corresponding transmission decoding or processing and/or de-packaging to obtain the encoded picture data 21.

Both, communication interface 22 and communication interface 28 may be configured as unidirectional communication interfaces as indicated by the arrow for the communication channel 13 in FIG. 1A pointing from the source device 12 to the destination device 14, or bi-directional communication interfaces, and may be configured, e.g. to send and receive messages, e.g. to set up a connection, to acknowledge and exchange any other information related to the communication link and/or data transmission, e.g. encoded picture data transmission.

The decoder 30 is configured to receive the encoded picture data 21 and provide decoded picture data 31 or a decoded picture 31 (further details will be described below, e.g., based on FIG. 3 or FIG. 5 ).

The post-processor 32 of destination device 14 is configured to post-process the decoded picture data 31 (also called reconstructed picture data), e.g. the decoded picture 31, to obtain post-processed picture data 33, e.g. a post-processed picture 33. The post-processing performed by the post-processing unit 32 may comprise, e.g. color format conversion (e.g. from YCbCr to RGB), color correction, trimming, or re-sampling, or any other processing, e.g. for preparing the decoded picture data 31 for display, e.g. by display device 34.

The display device 34 of the destination device 14 is configured to receive the post-processed picture data 33 for displaying the picture, e.g. to a user or viewer. The display device 34 may be or comprise any kind of display for representing the reconstructed picture, e.g. an integrated or external display or monitor. The displays may, e.g. comprise liquid crystal displays (LCD), organic light emitting diodes (OLED) displays, plasma displays, projectors, micro LED displays, liquid crystal on silicon (LCoS), digital light processor (DLP) or any kind of other display.

Although FIG. 1A depicts the source device 12 and the destination device 14 as separate devices, embodiments of devices may also comprise both or both functionalities, the source device 12 or corresponding functionality and the destination device 14 or corresponding functionality. In such embodiments the source device 12 or corresponding functionality and the destination device 14 or corresponding functionality may be implemented using the same hardware and/or software or by separate hardware and/or software or any combination thereof.

As will be apparent for the skilled person based on the description, the existence and (exact) split of functionalities of the different units or functionalities within the source device 12 and/or destination device 14 as shown in FIG. 1A may vary depending on the actual device and application.

The encoder 20 (e.g. a video encoder 20) or the decoder 30 (e.g. a video decoder 30) or both encoder 20 and decoder 30 may be implemented via processing circuitry as shown in FIG. 1B, such as one or more microprocessors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), discrete logic, hardware, video coding dedicated or any combinations thereof. The encoder 20 may be implemented via processing circuitry 46 to embody the various modules as discussed with respect to encoder 20 of FIG. 2 and/or any other encoder system or subsystem described herein. The decoder 30 may be implemented via processing circuitry 46 to embody the various modules as discussed with respect to decoder 30 of FIG. 3 and/or any other decoder system or subsystem described herein. The processing circuitry may be configured to perform the various operations as discussed later. As shown in FIG. 5 , if the techniques are implemented partially in software, a device may store instructions for the software in a suitable, non-transitory computer-readable storage medium and may execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Either of video encoder 20 and video decoder 30 may be integrated as part of a combined encoder/decoder (CODEC) in a single device, for example, as shown in FIG. 1B.

Source device 12 and destination device 14 may comprise any of a wide range of devices, including any kind of handheld or stationary devices, e.g. notebook or laptop computers, mobile phones, smart phones, tablets or tablet computers, cameras, desktop computers, set-top boxes, televisions, display devices, digital media players, video gaming consoles, video streaming devices (such as content services servers or content delivery servers), broadcast receiver device, broadcast transmitter device, or the like and may use no or any kind of operating system. In some cases, the source device 12 and the destination device 14 may be equipped for wireless communication. Thus, the source device 12 and the destination device 14 may be wireless communication devices.

In some cases, video coding system 10 illustrated in FIG. 1A is merely an example and the techniques of the present application may apply to video coding settings (e.g., video encoding or video decoding) that do not necessarily include any data communication between the encoding and decoding devices. In other examples, data is retrieved from a local memory, streamed over a network, or the like. A video encoding device may encode and store data to memory, and/or a video decoding device may retrieve and decode data from memory. In some examples, the encoding and decoding is performed by devices that do not communicate with one another, but simply encode data to memory and/or retrieve and decode data from memory.

For convenience of description, embodiments of the invention are described herein, for example, by reference to High-Efficiency Video Coding (HEVC) or to the reference software of Versatile Video coding (VVC), the next generation video coding standard developed by the Joint Collaboration Team on Video Coding (JCT-VC) of ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). One of ordinary skill in the art will understand that embodiments of the invention are not limited to HEVC or VVC.

Encoder and Encoding Method

FIG. 2 shows a schematic block diagram of an example video encoder 20 that is configured to implement the techniques of the present application. In the example of FIG. 2 , the video encoder 20 comprises an input 201 (or input interface 201), a residual calculation unit 204, a transform processing unit 206, a quantization unit 208, an inverse quantization unit 210, and inverse transform processing unit 212, a reconstruction unit 214, a loop filter unit 220, a decoded picture buffer (DPB) 230, a mode selection unit 260, an entropy encoding unit 270 and an output 272 (or output interface 272). The mode selection unit 260 may include an inter prediction unit 244, an intra prediction unit 254 and a partitioning unit 262. Inter prediction unit 244 may include a motion estimation unit and a motion compensation unit (not shown). A video encoder 20 as shown in FIG. 2 may also be referred to as hybrid video encoder or a video encoder according to a hybrid video codec.

The residual calculation unit 204, the transform processing unit 206, the quantization unit 208, the mode selection unit 260 may be referred to as forming a forward signal path of the encoder 20, whereas the inverse quantization unit 210, the inverse transform processing unit 212, the reconstruction unit 214, the buffer 216, the loop filter 220, the decoded picture buffer (DPB) 230, the inter prediction unit 244 and the intra-prediction unit 254 may be referred to as forming a backward signal path of the video encoder 20, wherein the backward signal path of the video encoder 20 corresponds to the signal path of the decoder (see video decoder 30 in FIG. 3 ). The inverse quantization unit 210, the inverse transform processing unit 212, the reconstruction unit 214, the loop filter 220, the decoded picture buffer (DPB) 230, the inter prediction unit 244 and the intra-prediction unit 254 are also referred to forming the “built-in decoder” of video encoder 20.

Pictures & Picture Partitioning (Pictures & Blocks)

The encoder 20 may be configured to receive, e.g. via input 201, a picture 17 (or picture data 17), e.g. picture of a sequence of pictures forming a video or video sequence. The received picture or picture data may also be a pre-processed picture 19 (or pre-processed picture data 19). For sake of simplicity the following description refers to the picture 17. The picture 17 may also be referred to as current picture or picture to be coded (in particular in video coding to distinguish the current picture from other pictures, e.g. previously encoded and/or decoded pictures of the same video sequence, i.e. the video sequence which also comprises the current picture).

A (digital) picture is or can be regarded as a two-dimensional array or matrix of samples with intensity values. A sample in the array may also be referred to as pixel (short form of picture element) or a pel. The number of samples in horizontal and vertical direction (or axis) of the array or picture define the size and/or resolution of the picture. For representation of color, typically three color components are employed, i.e. the picture may be represented or include three sample arrays. In RBG format or color space a picture comprises a corresponding red, green and blue sample array. However, in video coding each pixel is typically represented in a luminance and chrominance format or color space, e.g. YCbCr, which comprises a luminance component indicated by Y (sometimes also L is used instead) and two chrominance components indicated by Cb and Cr. The luminance (or short luma) component Y represents the brightness or grey level intensity (e.g. like in a grey-scale picture), while the two chrominance (or short chroma) components Cb and Cr represent the chromaticity or color information components. Accordingly, a picture in YCbCr format comprises a luminance sample array of luminance sample values (Y), and two chrominance sample arrays of chrominance values (Cb and Cr). Pictures in RGB format may be converted or transformed into YCbCr format and vice versa, the process is also known as color transformation or conversion. If a picture is monochrome, the picture may comprise only a luminance sample array. Accordingly, a picture may be, for example, an array of luma samples in monochrome format or an array of luma samples and two corresponding arrays of chroma samples in 4:2:0, 4:2:2, and 4:4:4 colour format.

Embodiments of the video encoder 20 may comprise a picture partitioning unit (not depicted in FIG. 2 ) configured to partition the picture 17 into a plurality of (typically non-overlapping) picture blocks 203. These blocks may also be referred to as root blocks, macro blocks (H.264/AVC) or coding tree blocks (CTB) or coding tree units (CTU) (H.265/HEVC and VVC). The picture partitioning unit may be configured to use the same block size for all pictures of a video sequence and the corresponding grid defining the block size, or to change the block size between pictures or subsets or groups of pictures, and partition each picture into the corresponding blocks.

In further embodiments, the video encoder may be configured to receive directly a block 203 of the picture 17, e.g. one, several or all blocks forming the picture 17. The picture block 203 may also be referred to as current picture block or picture block to be coded.

Like the picture 17, the picture block 203 again is or can be regarded as a two-dimensional array or matrix of samples with intensity values (sample values), although of smaller dimension than the picture 17. In other words, the block 203 may comprise, e.g., one sample array (e.g. a luma array in case of a monochrome picture 17, or a luma or chroma array in case of a color picture) or three sample arrays (e.g. a luma and two chroma arrays in case of a color picture 17) or any other number and/or kind of arrays depending on the color format applied. The number of samples in horizontal and vertical direction (or axis) of the block 203 define the size of block 203. Accordingly, a block may, for example, an M×N (M-column by N-row) array of samples, or an M×N array of transform coefficients.

Embodiments of the video encoder 20 as shown in FIG. 2 may be configured to encode the picture 17 block by block, e.g. the encoding and prediction is performed per block 203.

Embodiments of the video encoder 20 as shown in FIG. 2 may be further configured to partition and/or encode the picture by using slices (also referred to as video slices), wherein a picture may be partitioned into or encoded using one or more slices (typically non-overlapping), and each slice may comprise one or more blocks (e.g. CTUs).

Embodiments of the video encoder 20 as shown in FIG. 2 may be further configured to partition and/or encode the picture by using tile groups (also referred to as video tile groups) and/or tiles (also referred to as video tiles), wherein a picture may be partitioned into or encoded using one or more tile groups (typically non-overlapping), and each tile group may comprise, e.g. one or more blocks (e.g. CTUs) or one or more tiles, wherein each tile, e.g. may be of rectangular shape and may comprise one or more blocks (e.g. CTUs), e.g. complete or fractional blocks.

Residual Calculation

The residual calculation unit 204 may be configured to calculate a residual block 205 (also referred to as residual 205) based on the picture block 203 and a prediction block 265 (further details about the prediction block 265 are provided later), e.g. by subtracting sample values of the prediction block 265 from sample values of the picture block 203, sample by sample (pixel by pixel) to obtain the residual block 205 in the sample domain.

Transform

The transform processing unit 206 may be configured to apply a transform, e.g. a discrete cosine transform (DCT) or discrete sine transform (DST), on the sample values of the residual block 205 to obtain transform coefficients 207 in a transform domain. The transform coefficients 207 may also be referred to as transform residual coefficients and represent the residual block 205 in the transform domain.

The transform processing unit 206 may be configured to apply integer approximations of DCT/DST, such as the transforms specified for H.265/HEVC. Compared to an orthogonal DCT transform, such integer approximations are typically scaled by a certain factor. In order to preserve the norm of the residual block which is processed by forward and inverse transforms, additional scaling factors are applied as part of the transform process. The scaling factors are typically chosen based on certain constraints like scaling factors being a power of two for shift operations, bit depth of the transform coefficients, tradeoff between accuracy and implementation costs, etc. Specific scaling factors are, for example, specified for the inverse transform, e.g. by inverse transform processing unit 212 (and the corresponding inverse transform, e.g. by inverse transform processing unit 312 at video decoder 30) and corresponding scaling factors for the forward transform, e.g. by transform processing unit 206, at an encoder 20 may be specified accordingly.

Embodiments of the video encoder 20 (respectively transform processing unit 206) may be configured to output transform parameters, e.g. a type of transform or transforms, e.g. directly or encoded or compressed via the entropy encoding unit 270, so that, e.g., the video decoder 30 may receive and use the transform parameters for decoding.

Quantization

The quantization unit 208 may be configured to quantize the transform coefficients 207 to obtain quantized coefficients 209, e.g. by applying scalar quantization or vector quantization. The quantized coefficients 209 may also be referred to as quantized transform coefficients 209 or quantized residual coefficients 209.

The quantization process may reduce the bit depth associated with some or all of the transform coefficients 207. For example, an n-bit transform coefficient may be rounded down to an m-bit Transform coefficient during quantization, where n is greater than m. The degree of quantization may be modified by adjusting a quantization parameter (QP). For example for scalar quantization, different scaling may be applied to achieve finer or coarser quantization. Smaller quantization step sizes correspond to finer quantization, whereas larger quantization step sizes correspond to coarser quantization. The applicable quantization step size may be indicated by a quantization parameter (QP). The quantization parameter may for example be an index to a predefined set of applicable quantization step sizes. For example, small quantization parameters may correspond to fine quantization (small quantization step sizes) and large quantization parameters may correspond to coarse quantization (large quantization step sizes) or vice versa. The quantization may include division by a quantization step size and a corresponding and/or the inverse dequantization, e.g. by inverse quantization unit 210, may include multiplication by the quantization step size. Embodiments according to some standards, e.g. HEVC, may be configured to use a quantization parameter to determine the quantization step size. Generally, the quantization step size may be calculated based on a quantization parameter using a fixed point approximation of an equation including division. Additional scaling factors may be introduced for quantization and dequantization to restore the norm of the residual block, which might get modified because of the scaling used in the fixed point approximation of the equation for quantization step size and quantization parameter. In one example implementation, the scaling of the inverse transform and dequantization might be combined. Alternatively, customized quantization tables may be used and signaled from an encoder to a decoder, e.g. in a bitstream. The quantization is a lossy operation, wherein the loss increases with increasing quantization step sizes.

A picture compression level is controlled by quantization parameter (QP) that may be fixed for the whole picture (e.g. by using a same quantization parameter value), or may have different quantization parameter values for different regions of the picture.

For YCbCr 4:2:0 and 4:2:2 video, the signal characteristics of luma and the chroma

-   -   components are quite different. Specifically, chroma often         exhibits a strong lowpass     -   character. If strong quantization is applied, the chroma         information may be completely quantized to zero, which would         lead to the complete loss of color. Accordingly, in order to         reduce this the quantizer step size for chroma is adapted by         reducing the chroma quantizer step size for high QP values [2]         by applying chroma QP mapping function.

In the High Efficiency Video Coding (HEVC) standard as specified in [1], the chroma quantization parameter QPc is derived by Table 1, where qPi is equal to the associated luma quantization parameter plus the chroma QP offset value signaled in the picture parameter set (PPS) and/or the slice header. The derivation of chroma QP value from the associated luma QP value can be adjusted by signaling different chroma QP offset values. A positive chroma QP offset value will result in a coarser quantizer for the associated chroma component.

TABLE 1 Specification of QpC as a function of qPi in HEVC as an example qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 Qp_(C) =qPi 29 30 31 32 33 33 34 35 35 35 36 36 37 37 =qPi − 6

Schematic presentation of the mapping of the luma quantization parameter QP1 to the chroma quantization parameter QPc for HEVC (black) and H.264|AVC (gray) is presented on FIG. 6 according to [2].

In HEVC standard, a QP value for Luminance (or Luma) coding block (CB) is derived based on Predicted QP (qPY_PRED), which in its turn depends on CB location in frame/slice/tile. Then Qp_(Y) variable is derived by following Equation 1: Qp _(Y)=((qPY _(PRED) +CuQpDeltaVal+64+2*QpBdOffsetY)%(64+QpBdOffsetY))−QpBdOffsetY   (Equation 1) Wherein CuQpDeltaVal is a delta QP value which is signaled or derived for a coding unit (CU); QpBdOffsetY is a constant offset depended on Luma bit depth (From HEVC standard, this term corresponds to “the bit depth of the samples of the luma array”). Finally, quantization parameter Qp_(Y)′ of the Luminance (or Luma) component may be calculated by following Equation 2: Qp _(Y) ′=Qp _(Y) +QpBdOffsetY  (Equation 2)

The variables qPCb and qPCr are set equal to the value of QpC as specified in a mapping table (e.g. Table 1) based on the index qPi equal to qPiCb or qPiCr, respectively, and qPiCb and qPiCr are derived as follows by Equation 3: qPi _(Cb)=Clip3(−QpBdOffsetC,69,Qp _(Y)+pps_cb_qp_offset+slice_cb_qp_offset) qPi _(Cr)=Clip3(−QpBdOffsetC,69,Qp _(Y)+pps_cr_qp_offset+slice_cr_qp_offset)   (Equation 3) Wherein QpBdOffsetC is a constant offset depended on Chroma bit depth (From HEVC standard, this term corresponds to “the bit depth of the samples of the Chroma array”); pps_cb_qp_offset or pps_cr_qp_offset is a fixed offset for Cb component or Cr component signaled by a picture parameter set (PPS), and slice_cb_qp_offset or slice_cr_qp_offset is a fixed offset for Cb component or Cr component which is signaled in a slice header.

$\begin{matrix} {{{Clip}\mspace{11mu} 3\left( {x,y,z} \right)} = \left\{ \begin{matrix} {x;{z < x}} \\ {y;{z > y}} \\ {z;\ {otherwise}} \end{matrix} \right.} & {{Equation}\mspace{14mu} 4} \end{matrix}$

The Chroma quantization parameters for the Cb and Cr components (Qp_(Cb)′ and Qp_(Cr)′, are derived as follows Equation 5: QP _(Cb) ′=qP _(Cb) +QpBdOffsetC Qp _(Cr) ′=qP _(Cr) +QpBdOffsetC   Equation 5

The variables qP_(Cb) and qP_(Cr) are set equal to the value of Qp_(C) as specified in Table 1 based on the index qPi equal to qPi_(Cb) and qPi_(Cr), respectively.

VVC is a newly developing standard in specification draft 5 contains the following procedure to derive chroma quantization parameter:

-   -   When treeType is equal to DUAL_TREE_CHROMA, the variable Qp_(Y)         is set equal to the luma quantization parameter Qp_(Y) of the         luma coding unit that covers the luma location (xCb+cbWidth/2,         yCb+cbHeight/2).     -   The variables qP_(Cb), qP_(Cr) and qP_(Cbcr) are derived as         follows:         qPi _(Cb)=Clip3(−QpBdOffset_(C),69,Qp         _(Y)+pps_cb_qp_offset+slice_cb_qp_offset)  (8-926)         qPi _(Cr)=Clip3(−QpBdOffset_(C),69,Qp         _(Y)+pps_cr_qp_offset+slice_cr_qp_offset)  (8-927)         qPi _(CbCr)=Clip3(−QpBdOffset_(C),69,Qp         _(Y)+pps_joint_cbcr_qp_offset+slice_joint_cbcr_qp_offset)  (8-928)     -   If ChromaArrayType is equal to 1, the variables qP_(Cb), qP_(Cr)         and qP_(CbCr) are set equal to the value of Qp_(C) as specified         in         Qp′CbCr=qPCbCr+QpBdOffsetC  (8931)     -   Table 2 based on the index qPi equal to qPi_(Cb), qPi_(Cr) and         qPi_(CbCr), respectively.     -   Otherwise, the variables qP_(Cb), qP_(Cr) and qP_(CbCr) are set         equal to Min(qPi, 63), based on the index qPi equal to qPi_(Cb),         qPi_(Cr) and qPi_(CbCr), respectively.     -   The chroma quantization parameters for the Cb and Cr components,         Qp′_(Cb) and Qp′_(Cr), and joint Cb-Cr coding Qp′_(CbCr) are         derived as follows:         Qp′ _(Cb) =qP _(Cb) +QpBdOffset_(C)  (8-929)         QP′ _(Cr) =qP _(Cr) +QpBdOffset_(C)  (8-930)         QP′ _(CbCr) =qP _(CbCr) +QpBdOffset_(C)  (8-931)

TABLE 2 Specification of QpC as a function of qPi for ChromaArrayType equal to 1 qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 Qp_(C) =qPi 29 30 31 32 33 33 34 34 35 35 36 36 37 37 =qPi − 6

It can be seen that same chroma Qp mapping table (Table 2) as in HEVC standard (Table 1) is used. In contrast to HEVC, Qp′CbCr is introduced besides of Qp′Cb and Qp′Cr to derive quantization parameters for blocks were Cb and Cr color components are jointly quantized. This Qp′CbCr parameter is also derived based on chroma Qp mapping function specified in Table 2.

Since chroma compression efficiency has been significantly improved in VVC due to including chroma dedicated coding tools like chroma separate tree and CCLM, the chroma Qp mapping function designed for previous HEVC standard may be not optimal and may require adjustment. This disclosure proposed an optimized solution for the derivation of a chroma QP, which further explores the relationship between the luma QP and Chroma QP and take its advantage to improve the compression performance. The disclosure will be detailed later.

Embodiments of the video encoder 20 (respectively quantization unit 208) may be configured to output quantization parameters (QP), e.g. directly or encoded via the entropy encoding unit 270, so that, e.g., the video decoder 30 may receive and apply the quantization parameters for decoding.

Inverse Quantization

The inverse quantization unit 210 is configured to apply the inverse quantization of the quantization unit 208 on the quantized coefficients to obtain dequantized coefficients 211, e.g. by applying the inverse of the quantization scheme applied by the quantization unit 208 based on or using the same quantization step size as the quantization unit 208. The dequantized coefficients 211 may also be referred to as dequantized residual coefficients 211 and correspond—although typically not identical to the transform coefficients due to the loss by quantization—to the transform coefficients 207.

Inverse Transform

The inverse transform processing unit 212 is configured to apply the inverse transform of the transform applied by the transform processing unit 206, e.g. an inverse discrete cosine transform (DCT) or inverse discrete sine transform (DST) or other inverse transforms, to obtain a reconstructed residual block 213 (or corresponding dequantized coefficients 213) in the sample domain. The reconstructed residual block 213 may also be referred to as transform block 213.

Reconstruction

The reconstruction unit 214 (e.g. adder or summer 214) is configured to add the transform block 213 (i.e. reconstructed residual block 213) to the prediction block 265 to obtain a reconstructed block 215 in the sample domain, e.g. by adding—sample by sample—the sample values of the reconstructed residual block 213 and the sample values of the prediction block 265.

Filtering

The loop filter unit 220 (or short “loop filter” 220), is configured to filter the reconstructed block 215 to obtain a filtered block 221, or in general, to filter reconstructed samples to obtain filtered samples. The loop filter unit is, e.g., configured to smooth pixel transitions, or otherwise improve the video quality. The loop filter unit 220 may comprise one or more loop filters such as a de-blocking filter, a sample-adaptive offset (SAO) filter or one or more other filters, e.g. a bilateral filter, an adaptive loop filter (ALF), a sharpening, a smoothing filters or a collaborative filters, or any combination thereof. Although the loop filter unit 220 is shown in FIG. 2 as being an in loop filter, in other configurations, the loop filter unit 220 may be implemented as a post loop filter. The filtered block 221 may also be referred to as filtered reconstructed block 221.

Embodiments of the video encoder 20 (respectively loop filter unit 220) may be configured to output loop filter parameters (such as sample adaptive offset information), e.g. directly or encoded via the entropy encoding unit 270, so that, e.g., a decoder 30 may receive and apply the same loop filter parameters or respective loop filters for decoding.

Decoded Picture Buffer

The decoded picture buffer (DPB) 230 may be a memory that stores reference pictures, or in general reference picture data, for encoding video data by video encoder 20. The DPB 230 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. The decoded picture buffer (DPB) 230 may be configured to store one or more filtered blocks 221. The decoded picture buffer 230 may be further configured to store other previously filtered blocks, e.g. previously reconstructed and filtered blocks 221, of the same current picture or of different pictures, e.g. previously reconstructed pictures, and may provide complete previously reconstructed, i.e. decoded, pictures (and corresponding reference blocks and samples) and/or a partially reconstructed current picture (and corresponding reference blocks and samples), for example for inter prediction. The decoded picture buffer (DPB) 230 may be also configured to store one or more unfiltered reconstructed blocks 215, or in general unfiltered reconstructed samples, e.g. if the reconstructed block 215 is not filtered by loop filter unit 220, or any other further processed version of the reconstructed blocks or samples.

Mode Selection (Partitioning & Prediction)

The mode selection unit 260 comprises partitioning unit 262, inter-prediction unit 244 and intra-prediction unit 254, and is configured to receive or obtain original picture data, e.g. an original block 203 (current block 203 of the current picture 17), and reconstructed picture data, e.g. filtered and/or unfiltered reconstructed samples or blocks of the same (current) picture and/or from one or a plurality of previously decoded pictures, e.g. from decoded picture buffer 230 or other buffers (e.g. line buffer, not shown). The reconstructed picture data is used as reference picture data for prediction, e.g. inter-prediction or intra-prediction, to obtain a prediction block 265 or predictor 265.

Mode selection unit 260 may be configured to determine or select a partitioning for a current block prediction mode (including no partitioning) and a prediction mode (e.g. an intra or inter prediction mode) and generate a corresponding prediction block 265, which is used for the calculation of the residual block 205 and for the reconstruction of the reconstructed block 215.

Embodiments of the mode selection unit 260 may be configured to select the partitioning and the prediction mode (e.g. from those supported by or available for mode selection unit 260), which provide the best match or in other words the minimum residual (minimum residual means better compression for transmission or storage), or a minimum signaling overhead (minimum signaling overhead means better compression for transmission or storage), or which considers or balances both. The mode selection unit 260 may be configured to determine the partitioning and prediction mode based on rate distortion optimization (RDO), i.e. select the prediction mode which provides a minimum rate distortion. Terms like “best”, “minimum”, “optimum” etc. in this context do not necessarily refer to an overall “best”, “minimum”, “optimum”, etc. but may also refer to the fulfillment of a termination or selection criterion like a value exceeding or falling below a threshold or other constraints leading potentially to a “sub-optimum selection” but reducing complexity and processing time.

In other words, the partitioning unit 262 may be configured to partition the block 203 into smaller block partitions or sub-blocks (which form again blocks), e.g. iteratively using quad-tree-partitioning (QT), binary partitioning (BT) or triple-tree-partitioning (TT) or any combination thereof, and to perform, e.g., the prediction for each of the block partitions or sub-blocks, wherein the mode selection comprises the selection of the tree-structure of the partitioned block 203 and the prediction modes are applied to each of the block partitions or sub-blocks.

In the following the partitioning (e.g. by partitioning unit 260) and prediction processing (by inter-prediction unit 244 and intra-prediction unit 254) performed by an example video encoder 20 will be explained in more detail.

Partitioning

The partitioning unit 262 may partition (or split) a current block 203 into smaller partitions, e.g. smaller blocks of square or rectangular size. These smaller blocks (which may also be referred to as sub-blocks) may be further partitioned into even smaller partitions. This is also referred to tree-partitioning or hierarchical tree-partitioning, wherein a root block, e.g. at root tree-level 0 (hierarchy-level 0, depth 0), may be recursively partitioned, e.g. partitioned into two or more blocks of a next lower tree-level, e.g. nodes at tree-level 1 (hierarchy-level 1, depth 1), wherein these blocks may be again partitioned into two or more blocks of a next lower level, e.g. tree-level 2 (hierarchy-level 2, depth 2), etc. until the partitioning is terminated, e.g. because a termination criterion is fulfilled, e.g. a maximum tree depth or minimum block size is reached. Blocks which are not further partitioned are also referred to as leaf-blocks or leaf nodes of the tree. A tree using partitioning into two partitions is referred to as binary-tree (BT), a tree using partitioning into three partitions is referred to as ternary-tree (TT), and a tree using partitioning into four partitions is referred to as quad-tree (QT).

As mentioned before, the term “block” as used herein may be a portion, in particular a square or rectangular portion, of a picture. With reference, for example, to HEVC and VVC, the block may be or correspond to a coding tree unit (CTU), a coding unit (CU), prediction unit (PU), and transform unit (TU) and/or to the corresponding blocks, e.g. a coding tree block (CTB), a coding block (CB), a transform block (TB) or prediction block (PB).

For example, a coding tree unit (CTU) may be or comprise a CTB of luma samples, two corresponding CTBs of chroma samples of a picture that has three sample arrays, or a CTB of samples of a monochrome picture or a picture that is coded using three separate colour planes and syntax structures used to code the samples. Correspondingly, a coding tree block (CTB) may be an N×N block of samples for some value of N such that the division of a component into CTBs is a partitioning. A coding unit (CU) may be or comprise a coding block of luma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate colour planes and syntax structures used to code the samples. Correspondingly a coding block (CB) may be an M×N block of samples for some values of M and N such that the division of a CTB into coding blocks is a partitioning.

In embodiments, e.g., according to HEVC, a coding tree unit (CTU) may be split into CUs by using a quad-tree structure denoted as coding tree. The decision whether to code a picture area using inter-picture (temporal) or intra-picture (spatial) prediction is made at the CU level. Each CU can be further split into one, two or four PUs according to the PU splitting type. Inside one PU, the same prediction process is applied and the relevant information is transmitted to the decoder on a PU basis. After obtaining the residual block by applying the prediction process based on the PU splitting type, a CU can be partitioned into transform units (TUs) according to another quadtree structure similar to the coding tree for the CU.

In embodiments, e.g., according to the latest video coding standard currently in development, which is referred to as Versatile Video Coding (VVC), a combined Quad-tree and binary tree (QTBT) partitioning is for example used to partition a coding block. In the QTBT block structure, a CU can have either a square or rectangular shape. For example, a coding tree unit (CTU) is first partitioned by a quadtree structure. The quadtree leaf nodes are further partitioned by a binary tree or ternary (or triple) tree structure. The partitioning tree leaf nodes are called coding units (CUs), and that segmentation is used for prediction and transform processing without any further partitioning. This means that the CU, PU and TU have the same block size in the QTBT coding block structure. In parallel, multiple partition, for example, triple tree partition may be used together with the QTBT block structure.

In one example, the mode selection unit 260 of video encoder 20 may be configured to perform any combination of the partitioning techniques described herein.

As described above, the video encoder 20 is configured to determine or select the best or an optimum prediction mode from a set of (e.g. pre-determined) prediction modes. The set of prediction modes may comprise, e.g., intra-prediction modes and/or inter-prediction modes.

Intra-Prediction

The set of intra-prediction modes may comprise 35 different intra-prediction modes, e.g. non-directional modes like DC (or mean) mode and planar mode, or directional modes, e.g. as defined in HEVC, or may comprise 67 different intra-prediction modes, e.g. non-directional modes like DC (or mean) mode and planar mode, or directional modes, e.g. as defined for VVC.

The intra-prediction unit 254 is configured to use reconstructed samples of neighboring blocks of the same current picture to generate an intra-prediction block 265 according to an intra-prediction mode of the set of intra-prediction modes.

The intra prediction unit 254 (or in general the mode selection unit 260) is further configured to output intra-prediction parameters (or in general information indicative of the selected intra prediction mode for the block) to the entropy encoding unit 270 in form of syntax elements 266 for inclusion into the encoded picture data 21, so that, e.g., the video decoder 30 may receive and use the prediction parameters for decoding.

Inter-Prediction

The set of (or possible) inter-prediction modes depends on the available reference pictures (i.e. previous at least partially decoded pictures, e.g. stored in DBP 230) and other inter-prediction parameters, e.g. whether the whole reference picture or only a part, e.g. a search window area around the area of the current block, of the reference picture is used for searching for a best matching reference block, and/or e.g. whether pixel interpolation is applied, e.g. half/semi-pel and/or quarter-pel interpolation, or not.

Additional to the above prediction modes, skip mode and/or direct mode may be applied.

The inter prediction unit 244 may include a motion estimation (ME) unit and a motion compensation (MC) unit (both not shown in FIG. 2 ). The motion estimation unit may be configured to receive or obtain the picture block 203 (current picture block 203 of the current picture 17) and a decoded picture 231, or at least one or a plurality of previously reconstructed blocks, e.g. reconstructed blocks of one or a plurality of other/different previously decoded pictures 231, for motion estimation. E.g. a video sequence may comprise the current picture and the previously decoded pictures 231, or in other words, the current picture and the previously decoded pictures 231 may be part of or form a sequence of pictures forming a video sequence.

The encoder 20 may, e.g., be configured to select a reference block from a plurality of reference blocks of the same or different pictures of the plurality of other pictures and provide a reference picture (or reference picture index) and/or an offset (spatial offset) between the position (x, y coordinates) of the reference block and the position of the current block as inter prediction parameters to the motion estimation unit. This offset is also called motion vector (MV).

The motion compensation unit is configured to obtain, e.g. receive, an inter prediction parameter and to perform inter prediction based on or using the inter prediction parameter to obtain an inter prediction block 265. Motion compensation, performed by the motion compensation unit, may involve fetching or generating the prediction block based on the motion/block vector determined by motion estimation, possibly performing interpolations to sub-pixel precision. Interpolation filtering may generate additional pixel samples from known pixel samples, thus potentially increasing the number of candidate prediction blocks that may be used to code a picture block. Upon receiving the motion vector for the PU of the current picture block, the motion compensation unit may locate the prediction block to which the motion vector points in one of the reference picture lists.

The motion compensation unit may also generate syntax elements associated with the blocks and video slices for use by video decoder 30 in decoding the picture blocks of the video slice. In addition or as an alternative to slices and respective syntax elements, tile groups and/or tiles and respective syntax elements may be generated or used.

Entropy Coding

The entropy encoding unit 270 is configured to apply, for example, an entropy encoding algorithm or scheme (e.g. a variable length coding (VLC) scheme, an context adaptive VLC scheme (CAVLC), an arithmetic coding scheme, a binarization, a context adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy encoding methodology or technique) or bypass (no compression) on the quantized coefficients 209, inter prediction parameters, intra prediction parameters, loop filter parameters and/or other syntax elements to obtain encoded picture data 21 which can be output via the output 272, e.g. in the form of an encoded bitstream 21, so that, e.g., the video decoder 30 may receive and use the parameters for decoding. The encoded bitstream 21 may be transmitted to video decoder 30, or stored in a memory for later transmission or retrieval by video decoder 30.

Other structural variations of the video encoder 20 can be used to encode the video stream. For example, a non-transform based encoder 20 can quantize the residual signal directly without the transform processing unit 206 for certain blocks or frames. In another implementation, an encoder 20 can have the quantization unit 208 and the inverse quantization unit 210 combined into a single unit.

Decoder and Decoding Method

FIG. 3 shows an example of a video decoder 30 that is configured to implement the techniques of this present application. The video decoder 30 is configured to receive encoded picture data 21 (e.g. encoded bitstream 21), e.g. encoded by encoder 20, to obtain a decoded picture 331. The encoded picture data or bitstream comprises information for decoding the encoded picture data, e.g. data that represents picture blocks of an encoded video slice (and/or tile groups or tiles) and associated syntax elements.

In the example of FIG. 3 , the decoder 30 comprises an entropy decoding unit 304, an inverse quantization unit 310, an inverse transform processing unit 312, a reconstruction unit 314 (e.g. a summer 314), a loop filter 320, a decoded picture buffer (DBP) 330, a mode application unit 360, an inter prediction unit 344 and an intra prediction unit 354. Inter prediction unit 344 may be or include a motion compensation unit. Video decoder 30 may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder 100 from FIG. 2 .

As explained with regard to the encoder 20, the inverse quantization unit 210, the inverse transform processing unit 212, the reconstruction unit 214 the loop filter 220, the decoded picture buffer (DPB) 230, the inter prediction unit 344 and the intra prediction unit 354 are also referred to as forming the “built-in decoder” of video encoder 20. Accordingly, the inverse quantization unit 310 may be identical in function to the inverse quantization unit 110, the inverse transform processing unit 312 may be identical in function to the inverse transform processing unit 212, the reconstruction unit 314 may be identical in function to reconstruction unit 214, the loop filter 320 may be identical in function to the loop filter 220, and the decoded picture buffer 330 may be identical in function to the decoded picture buffer 230. Therefore, the explanations provided for the respective units and functions of the video 20 encoder apply correspondingly to the respective units and functions of the video decoder 30.

Entropy Decoding

The entropy decoding unit 304 is configured to parse the bitstream 21 (or in general encoded picture data 21) and perform, for example, entropy decoding to the encoded picture data 21 to obtain, e.g., quantized coefficients 309 and/or decoded coding parameters (not shown in FIG. 3 ), e.g. any or all of inter prediction parameters (e.g. reference picture index and motion vector), intra prediction parameter (e.g. intra prediction mode or index), transform parameters, quantization parameters, loop filter parameters, and/or other syntax elements. Entropy decoding unit 304 maybe configured to apply the decoding algorithms or schemes corresponding to the encoding schemes as described with regard to the entropy encoding unit 270 of the encoder 20. Entropy decoding unit 304 may be further configured to provide inter prediction parameters, intra prediction parameter and/or other syntax elements to the mode application unit 360 and other parameters to other units of the decoder 30. Video decoder 30 may receive the syntax elements at the video slice level and/or the video block level. In addition or as an alternative to slices and respective syntax elements, tile groups and/or tiles and respective syntax elements may be received and/or used.

Inverse Quantization

The inverse quantization unit 310 may be configured to receive quantization parameters (QP) (or in general information related to the inverse quantization) and quantized coefficients from the encoded picture data 21 (e.g. by parsing and/or decoding, e.g. by entropy decoding unit 304) and to apply based on the quantization parameters an inverse quantization on the decoded quantized coefficients 309 to obtain dequantized coefficients 311, which may also be referred to as transform coefficients 311. The inverse quantization process may include use of a quantization parameter determined by video encoder 20 for each video block in the video slice (or tile or tile group) to determine a degree of quantization and, likewise, a degree of inverse quantization that should be applied.

Inverse Transform

Inverse transform processing unit 312 may be configured to receive dequantized coefficients 311, also referred to as transform coefficients 311, and to apply a transform to the dequantized coefficients 311 in order to obtain reconstructed residual blocks 213 in the sample domain. The reconstructed residual blocks 213 may also be referred to as transform blocks 313. The transform may be an inverse transform, e.g., an inverse DCT, an inverse DST, an inverse integer transform, or a conceptually similar inverse transform process. The inverse transform processing unit 312 may be further configured to receive transform parameters or corresponding information from the encoded picture data 21 (e.g. by parsing and/or decoding, e.g. by entropy decoding unit 304) to determine the transform to be applied to the dequantized coefficients 311.

Reconstruction

The reconstruction unit 314 (e.g. adder or summer 314) may be configured to add the reconstructed residual block 313, to the prediction block 365 to obtain a reconstructed block 315 in the sample domain, e.g. by adding the sample values of the reconstructed residual block 313 and the sample values of the prediction block 365.

Filtering

The loop filter unit 320 (either in the coding loop or after the coding loop) is configured to filter the reconstructed block 315 to obtain a filtered block 321, e.g. to smooth pixel transitions, or otherwise improve the video quality. The loop filter unit 320 may comprise one or more loop filters such as a de-blocking filter, a sample-adaptive offset (SAO) filter or one or more other filters, e.g. a bilateral filter, an adaptive loop filter (ALF), a sharpening, a smoothing filters or a collaborative filters, or any combination thereof. Although the loop filter unit 320 is shown in FIG. 3 as being an in loop filter, in other configurations, the loop filter unit 320 may be implemented as a post loop filter.

Decoded Picture Buffer

The decoded video blocks 321 of a picture are then stored in decoded picture buffer 330, which stores the decoded pictures 331 as reference pictures for subsequent motion compensation for other pictures and/or for output respectively display.

The decoder 30 is configured to output the decoded picture 311, e.g. via output 312, for presentation or viewing to a user.

Prediction

The inter prediction unit 344 may be identical to the inter prediction unit 244 (in particular to the motion compensation unit) and the intra prediction unit 354 may be identical to the inter prediction unit 254 in function, and performs split or partitioning decisions and prediction based on the partitioning and/or prediction parameters or respective information received from the encoded picture data 21 (e.g. by parsing and/or decoding, e.g. by entropy decoding unit 304). Mode application unit 360 may be configured to perform the prediction (intra or inter prediction) per block based on reconstructed pictures, blocks or respective samples (filtered or unfiltered) to obtain the prediction block 365.

When the video slice is coded as an intra coded (I) slice, intra prediction unit 354 of mode application unit 360 is configured to generate prediction block 365 for a picture block of the current video slice based on a signaled intra prediction mode and data from previously decoded blocks of the current picture. When the video picture is coded as an inter coded (i.e., B, or P) slice, inter prediction unit 344 (e.g. motion compensation unit) of mode application unit 360 is configured to produce prediction blocks 365 for a video block of the current video slice based on the motion vectors and other syntax elements received from entropy decoding unit 304. For inter prediction, the prediction blocks may be produced from one of the reference pictures within one of the reference picture lists. Video decoder 30 may construct the reference frame lists, List 0 and List 1, using default construction techniques based on reference pictures stored in DPB 330. The same or similar may be applied for or by embodiments using tile groups (e.g. video tile groups) and/or tiles (e.g. video tiles) in addition or alternatively to slices (e.g. video slices), e.g. a video may be coded using I, P or B tile groups and/or tiles.

Mode application unit 360 is configured to determine the prediction information for a video block of the current video slice by parsing the motion vectors or related information and other syntax elements, and uses the prediction information to produce the prediction blocks for the current video block being decoded. For example, the mode application unit 360 uses some of the received syntax elements to determine a prediction mode (e.g., intra or inter prediction) used to code the video blocks of the video slice, an inter prediction slice type (e.g., B slice, P slice, or GPB slice), construction information for one or more of the reference picture lists for the slice, motion vectors for each inter encoded video block of the slice, inter prediction status for each inter coded video block of the slice, and other information to decode the video blocks in the current video slice. The same or similar may be applied for or by embodiments using tile groups (e.g. video tile groups) and/or tiles (e.g. video tiles) in addition or alternatively to slices (e.g. video slices), e.g. a video may be coded using I, P or B tile groups and/or tiles.

Embodiments of the video decoder 30 as shown in FIG. 3 may be configured to partition and/or decode the picture by using slices (also referred to as video slices), wherein a picture may be partitioned into or decoded using one or more slices (typically non-overlapping), and each slice may comprise one or more blocks (e.g. CTUs).

Embodiments of the video decoder 30 as shown in FIG. 3 may be configured to partition and/or decode the picture by using tile groups (also referred to as video tile groups) and/or tiles (also referred to as video tiles), wherein a picture may be partitioned into or decoded using one or more tile groups (typically non-overlapping), and each tile group may comprise, e.g. one or more blocks (e.g. CTUs) or one or more tiles, wherein each tile, e.g. may be of rectangular shape and may comprise one or more blocks (e.g. CTUs), e.g. complete or fractional blocks.

Other variations of the video decoder 30 can be used to decode the encoded picture data 21. For example, the decoder 30 can produce the output video stream without the loop filtering unit 320. For example, a non-transform based decoder 30 can inverse-quantize the residual signal directly without the inverse-transform processing unit 312 for certain blocks or frames. In another implementation, the video decoder 30 can have the inverse-quantization unit 310 and the inverse-transform processing unit 312 combined into a single unit.

It should be understood that, in the encoder 20 and the decoder 30, a processing result of a current step may be further processed and then output to the next step. For example, after interpolation filtering, motion vector derivation or loop filtering, a further operation, such as Clip or shift, may be performed on the processing result of the interpolation filtering, motion vector derivation or loop filtering.

It should be noted that further operations may be applied to the derived motion vectors of current block (including but not limit to control point motion vectors of affine mode, sub-block motion vectors in affine, planar, ATMVP modes, temporal motion vectors, and so on). For example, the value of motion vector is constrained to a predefined range according to its representing bit. If the representing bit of motion vector is bitDepth, then the range is −2{circumflex over ( )}(bitDepth−1)˜2{circumflex over ( )}(bitDepth−1)−1, where “{circumflex over ( )}” means exponentiation. For example, if bitDepth is set equal to 16, the range is −32768˜32767; if bitDepth is set equal to 18, the range is −131072˜131071. For example, the value of the derived motion vector (e.g. the MVs of four 4×4 sub-blocks within one 8×8 block) is constrained such that the max difference between integer parts of the four 4×4 sub-block MVs is no more than N pixels, such as no more than 1 pixel. Here provides two methods for constraining the motion vector according to the bitDepth.

Method 1: remove the overflow MSB (most significant bit) by flowing operations ux=(mvx+2^(bitDePth))% 2^(bitDepth)  (1) mvx=(ux>=2^(bitDepth-1))?(ux−2^(bitDepth)):ux  (2) uy=(mvy+2^(bitDepth))%2^(bitDepth)  (3) mvy=(uy>=2^(bitDepth-1))?(uy−2^(bitDepth)):uy  (4) where mvx is a horizontal component of a motion vector of an image block or a sub-block, mvy is a vertical component of a motion vector of an image block or a sub-block, and ux and uy indicates an intermediate value;

For example, if the value of mvx is −32769, after applying formula (1) and (2), the resulting value is 32767. In computer system, decimal numbers are stored as two's complement. The two's complement of −32769 is 1,0111,1111,1111,1111 (17 bits), then the MSB is discarded, so the resulting two's complement is 0111,1111,1111,1111 (decimal number is 32767), which is same as the output by applying formula (1) and (2). ux=(mvpx+mvdx+2^(bitDePth))%2^(bitDepth)  (5) mvx=(ux>=2^(bitDepth-1))?(ux−2^(bitDepth)):ux  (6) uy=(mvpy+mvdy+2^(bitDepth))%2^(bitDepth)  (7) mvy=(uy>=2^(bitDepth-1))?(uy−2^(bitDepth)):uy  (8)

The operations may be applied during the sum of mvp and mvd, as shown in formula (5) to (8).

Method 2: remove the overflow MSB by clipping the value vx=Clip3(−2^(bitDepth-1),2^(bitDepth-1)−1,vx) vy=Clip3(−2^(bitDepth-1),2^(bitDepth-1)−1,vy)

where vx is a horizontal component of a motion vector of an image block or a sub-block, vy is a vertical component of a motion vector of an image block or a sub-block; x, y and z respectively correspond to three input value of the MV clipping process, and the definition of function Clip3 is as follow:

${{Clip}\mspace{11mu} 3\left( {x,y,z} \right)} = \left\{ \begin{matrix} {x;} & {z < x} \\ {y;} & {z > y} \\ {z;} & {otherwise} \end{matrix} \right.$

FIG. 4 is a schematic diagram of a video coding device 400 according to an embodiment of the disclosure. The video coding device 400 is suitable for implementing the disclosed embodiments as described herein. In an embodiment, the video coding device 400 may be a decoder such as video decoder 30 of FIG. 1A or an encoder such as video encoder 20 of FIG. 1A.

The video coding device 400 comprises ingress ports 410 (or input ports 410) and receiver units (Rx) 420 for receiving data; a processor, logic unit, or central processing unit (CPU) 430 to process the data; transmitter units (Tx) 440 and egress ports 450 (or output ports 450) for transmitting the data; and a memory 460 for storing the data. The video coding device 400 may also comprise optical-to-electrical (OE) components and electrical-to-optical (EO) components coupled to the ingress ports 410, the receiver units 420, the transmitter units 440, and the egress ports 450 for egress or ingress of optical or electrical signals.

The processor 430 is implemented by hardware and software. The processor 430 may be implemented as one or more CPU chips, cores (e.g., as a multi-core processor), FPGAs, ASICs, and DSPs. The processor 430 is in communication with the ingress ports 410, receiver units 420, transmitter units 440, egress ports 450, and memory 460. The processor 430 comprises a coding module 470. The coding module 470 implements the disclosed embodiments described above. For instance, the coding module 470 implements, processes, prepares, or provides the various coding operations. The inclusion of the coding module 470 therefore provides a substantial improvement to the functionality of the video coding device 400 and effects a transformation of the video coding device 400 to a different state. Alternatively, the coding module 470 is implemented as instructions stored in the memory 460 and executed by the processor 430.

The memory 460 may comprise one or more disks, tape drives, and solid-state drives and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and to store instructions and data that are read during program execution. The memory 460 may be, for example, volatile and/or non-volatile and may be a read-only memory (ROM), random access memory (RAM), ternary content-addressable memory (TCAM), and/or static random-access memory (SRAM).

FIG. 5 is a simplified block diagram of an apparatus 500 that may be used as either or both of the source device 12 and the destination device 14 from FIG. 1 according to an exemplary embodiment.

A processor 502 in the apparatus 500 can be a central processing unit. Alternatively, the processor 502 can be any other type of device, or multiple devices, capable of manipulating or processing information now-existing or hereafter developed. Although the disclosed implementations can be practiced with a single processor as shown, e.g., the processor 502, advantages in speed and efficiency can be achieved using more than one processor.

A memory 504 in the apparatus 500 can be a read only memory (ROM) device or a random access memory (RAM) device in an implementation. Any other suitable type of storage device can be used as the memory 504. The memory 504 can include code and data 506 that is accessed by the processor 502 using a bus 512. The memory 504 can further include an operating system 508 and application programs 510, the application programs 510 including at least one program that permits the processor 502 to perform the methods described here. For example, the application programs 510 can include applications 1 through N, which further include a video coding application that performs the methods described here.

The apparatus 500 can also include one or more output devices, such as a display 518. The display 518 may be, in one example, a touch sensitive display that combines a display with a touch sensitive element that is operable to sense touch inputs. The display 518 can be coupled to the processor 502 via the bus 512.

Although depicted here as a single bus, the bus 512 of the apparatus 500 can be composed of multiple buses. Further, the secondary storage 514 can be directly coupled to the other components of the apparatus 500 or can be accessed via a network and can comprise a single integrated unit such as a memory card or multiple units such as multiple memory cards. The apparatus 500 can thus be implemented in a wide variety of configurations.

As described in M. Wien, “High Efficiency Video Coding—Coding Tools and Specification”, Springer, 2014 (reference [2]), the quantization process maps signal amplitudes to a predefined set of representative values. Quantization is an inherently non-linear lossy operation, which cannot be inverted. The quantization process is the part of the video coding scheme which inserts signal degradation and distortion by removing signal information from the coded representation. By careful control of the quantizer settings within the coded picture and over the coded video sequence, the amount of removed irrelevant information (i.e. information which can be removed without noticeable degradation) compared to the amount of removed relevant information, which leads to visible distortions, can be optimized. The number and values of the applicable quantizer step sizes in a video coding system is limited. The applicable quantizer step size is usually indicated by the quantization parameter (QP), which serves as an index to a predefined set of applicable quantization step sizes. Commonly, low quantization parameters correspond to fine quantization and high quantization parameters correspond to coarse quantizer step sizes. A high granularity of the quantizer step sizes is beneficial to allow for precise rate control in the encoded bitstream. On the other hand, the signaling for a high number of available quantizer step sizes induces additional coding cost, which needs to be considered. The quantization has to balance granularity and coding cost.

If strong quantization is applied, the chroma information may be completely quantized to zero, which would lead to the complete loss of color. Accordingly, in order to reduce this the quantizer step size for chroma is adapted by reducing the chroma quantizer step size for high QP values [2] by applying chroma QP mapping function.

As was mentioned above in the High Efficiency Video Coding (HEVC) and AVC standards quantizer step size for chroma is adapted by reducing the chroma quantizer step size for high QP values by applying chroma QP mapping function to prevent significant distortion or even complete diapering of chroma components in reconstructed signal.

The Versatile Video coding (VVC) VVC is a newly developing standard. During development several algorithms dedicated to chroma coding were included into the standard, such a Chroma Separate Tree (CST) and Cross-Component Linear Model (CCLM). That allowed to substantially improve compression efficiency for chroma components. Keep using chroma QP mapping table designed e.g. for HEVC leads to unbalanced compression performance across luma and chroma components. Since chroma performance is improved in VVC a new mapping table needs to be designed targeting to less QP attenuation for chroma component in higher QP range.

In HEVC chroma QP mapping table the maximum difference between QP index (QPi) derived based on luma QP and chroma QP (QPc) is set to 6 (see Table 2 for reference).

FIG. 6 illustrates the method according to the present disclosure. The method of obtaining quantization parameter (QP) for chrominance components based on QP for luminance component, wherein the method is performed by a decoder or encoder, comprises the following steps: obtaining a luminance QP for a luma component of a coding unit (Step 601); obtaining a QP index (QPi) based at least in a part on the luminance QP (step 602); and obtaining a chrominance QP (QPc) for a chroma component of the coding unit based on the QP index by using a preset lookup table, wherein the maximum difference between the QP index and the chrominance QP is equal to 3 (Step 603).

As indicated above, as along as the chrominance QP index is obtained, the obtained QP index may be clipped to a certain range as exemplified as follows. qPi _(Cb)=Clip3(−QpBdOffset_(C) ,A,Qp _(Y) +pps_cb_qp_offset+slice_cb_qp_offset)  (a) qPi _(Cr)=Clip3(−QpBdOffset_(C) ,A,Qp _(Y) +pps_cr_qp_offset+slice_cr_qp_offset)  (b) qPi _(CbCr)=Clip3(−QpBdOffset_(C) ,A,Qp _(Y) +pps_joint_cbcr_qp_offset+slice_joint_cbcr_qp_offset)   (c)

formulas (a), (b) and (c) standards for a clipping process for each of the QP index, wherein the qPi_(Cb), qPi_(Cr), and qPi_(CbCr) stand for the QP indices, which are clipped into the range from −QpBdOffsetC to A. A is a predefined constant value, which may be 57 or 69.

The obtained QP index is then to be used to derive the QP value for the chroma components of a coding block. The derivation of the QP value based on the QP index can be performed by using a equation or by using a lookup table that is detailed later.

The chroma QP value derived by the equation or by using the lookup table may be further modified by adding an offset as follows: Qp′ _(Cb) =qP _(Cb) +QpBdOffset_(C) Qp′ _(Cr) =qP _(Cr) +QpBdOffset_(C) Qp′ _(CbCr) =qP _(CbCr) +QpBdOffset_(C)

-   -   Qp′_(Cb) and Qp′_(Cr) and joint Cb-Cr coding Qp′_(CbCr) stands         for the chroma quantization parameters for the Cb and Cr         components. It should be noted that the joint Cb-Cr coding         Qp′_(CbCr) can be a flexible parameter which will not be used in         some cases. QpBdOffset_(C) stands for a offset that is used for         modifying the chroma QP value.

Therefore, the QP values obtained by applying the method according to the present disclosure may not be the final QP values, but respective offset values may be added.

As was described above chroma often exhibits a strong lowpass character. If strong quantization is applied, the chroma information may be completely quantized to zero, which would lead to the complete loss of color. Accordingly, in order to reduce this effect in practical application scenarios the quantizer step size for chroma is adapted by reducing the chroma quantizer step size for high QP values by applying chroma QP mapping function. However, the optimal attenuation for the QP for chroma components strongly depends on how efficient it is predicted and coded in specific video coding framework or video coding standard. In other words, it depends on amount and efficiency of coding tools included into coding framework. Since new VC standard under development contains new coding tools with better efficiency of chroma components coding the new relationship between QP index and the chroma QP value with more relaxed attenuation can be designed in order to achieve better chroma quality and/or better balancing between luma and chroma components quality. It should also be noted that changing relationship in quantization parameter setting for luma and chroma components may lead to redistribution of bits in the bitstream from luma to chroma components and visa versa. The suggested criterium for finding optimal relationship is the balanced coding efficiency between luma and chroma measured against some known reference, e.g. previous HEVC standard.

The following embodiment will be used to exemplify how the relationship between the QP index and the QP value should be set and how a lookup table may be designed.

In a first embodiment the maximum difference between QP index (QPi) derived based on luma QP and chroma QP (QPc) is set to 3, such as: QPi−QPc<=3

In a second embodiment the mapping function is defined as follows (also shown in FIG. 7 as lookup table 700):

QPi <35 35 36 37 38 39 40 41 42 43 >43 QP_(c) = QPi 34 35 36 37 37 38 39 40 40 = QPi − 3

By using this lookup table, the following results can be observed from the experiment. In this experiment we keep chroma QP Offset equal to 1 for AI configuration only. For configurations RA, LDB and LDP the chroma QP Offset is set to 0.

Here AI stands All Intra configuration when all frames in video sequence are coded as still pictures without referencing to other frames. Low Delay B (LDB) is a configuration intended for low delay applications such as real-time streaming. In this configuration only previous frames can be used as reference picture. Avoiding usage future frames allows to avoid additional algorithmic delay. In LDB configuration frames may contain Bi-predictive blocks (two references for a single block). The Low Delay P (LDP) is the same as LDB except of only P-predictive blocks (single reference) are allowed. This configuration is intended to be used for low-complexity real-time applications, since single reference prediction is twice simpler than bi-prediction. Random Access (RA) configuration is most efficient one and is used for general purpose video coding. The efficiency is achieved due to using past and future frames for prediction, wherein reference frames are organized in hierarchical pyramid. Besides of that AI frames are periodically (with interval roughly equal to 1 second) are coded to allow the decoder start decoding from this frame without decoding all previous pictures. By doing this the random access functionality is provided.

TABLE A Coding performance of adjusted mapping table over VTM5.0 (VTM-is a Test Model software for VVC) All Intra Main10 Proposal Over VTM5.0 Y U V EncT DecT Class A1 −1.17%  6.17%  4.96% 98% 100% Class A2 −1.69%  5.20%  5.14% 97%  99% Class B −0.63%  6.05%  6.77% 99% 101% Class C −0.69%  5.78%  6.00% 99% 101% Class E −0.62%  5.87%  6.34% 100%  101% Overall −0.91%  5.84%  5.95% 99% 100% Class D −0.67%  5.78%  6.16% 99%  99% Class F (optional) −0.75%  4.22%  4.24% 100%  100% Random Access Main 10 Proposal Over VTM5.0 Y U V EncT DecT Class A1 −2.34%  2.56% −0.45% 98% 100% Class A2 −1.18% −2.76% −2.97% 98% 100% Class B −0.21% −3.75% −3.11% 99% 100% Class C −0.38% −1.69% −0.76% 99% 101% Class E Overall −0.88% −1.74% −1.92% 99% 101% Class D −0.23% −2.44% −1.47% 100%  101% Class F (optional) −0.20% −2.26% −1.65% 99% 100% Low delay B Main10 Proposal Over VTM5.0 Y U V EncT DecT Class A1 Class A2 Class B  0.06% −8.04% −7.29% 100%  101% Class C  0.03% −4.25% −3.66% 100%   99% Class E  0.16% −7.41% −6.86% 99% 101% Overall  0.07% −6.62% −5.97% 100%  100% Class D  0.05% −6.00% −4.16% 101%  104% Class F (optional) −0.04% −3.20% −2.48% 99% 100% Low delay P Main10 Proposal Over VTM5.0 Y U V EncT DecT Class A1 Class A2 Class B  0.06% −8.18% −7.40% 99% 100% Class C −0.07% −4.69% −3.63% 99% 100% Class E  0.08% −7.54% −8.13% 100%  101% Overall  0.02% −6.86% −6.33% 99% 100% Class D  0.13% −6.53% −5.06% 100%  101% Class F (optional)  0.01% −2.69% −2.81% 100%  100%

It was noted that usage of VTM5.0 as an anchor leads crossing of RD curves for some sequences. That may make BD-rate numbers not relevant. To obtain correct numbers we estimate performance over VTM5.0 by taking difference in performance of VTM5.0 and test with adjusted mapping table over HM. On Table B below left part denotes the BD-rate difference with VTM5.0, middle part reports coding performance of adjusted table over HM16.20, and right part is coding performance of VTM5.0 over HM given for comparison.

TABLE B Difference in coding performance of adjusted mapping table over VTM5.0 having HM as an anchor All Intra Main10 Delta Over VTM-5.0 Proposal Over HM16.20 Y U V EncT DecT Y U V EncT DecT Class −0.92%  4.72%  3.72%  98% 100% −27.55% −35.08% −33.54% 2289% 211% A1 Class −1.47%  3.29%  4.35%  97%  99% −26.45% −22.54% −15.17% 3662% 220% A2 Class −0.53%  4.75%  5.38%  99% 101% −20.97% −20.13% −27.59% 3932% 225% B Class −0.58%  4.25%  4.48%  99% 101% −21.93% −17.92% −22.77% 5328% 232% C Class −0.51%  5.87%  6.34% 100% 101% −25.20% −19.51% −24.69% 3007% 217% E Over- −0.76%  4.45%  4.71%  99% 100% −23.90% −22.43% −24.96% 3632% 222% all Class −0.58%  5.16%  5.70%  99%  99% −18.04% −13.77% −15.95% 6149% 221% D Class −0.54%  2.50%  2.37% 100% 100% −38.69% −37.15% −39.71% 5290% 220% F (op- tional) VTM5.0 Over HM16.20 Y U V EncT DecT Class −26.64% −39.80% −37.26% 2277% 210% A1 Class −24.98% −25.83% −19.52% 3676% 220% A2 Class −20.45% −24.88% −32.98% 3872% 220% B Class −21.35% −22.17% −27.25% 5226% 222% C Class −24.70% −24.59% −29.92% 2928% 209% E Over- −23.14% −26.88% −29.67% 3585% 217% all Class −17.46% −18.93% −21.66% 6040% 213% D Class −38.15% −39.65% −42.08% 5124% 213% F (op- tional) Random Access Main 10 Delta Over VTM−5.0 Proposal Over HM16.20 Y U V EncT DecT Y U V EncT DecT Class −1.80%  3.34%  1.33%  98% 100% −36.71% −36.50% −41.69%  926% 231% A1 Class −1.09% −0.83% −1.26%  98% 100% −40.12% −37.86% −31.98% 1011% 236% A2 Class −0.20% −1.04% −0.45%  99% 100% −32.97% −38.92% −40.65% 1022% 226% B Class −0.36% −1.06% −0.11%  99% 101% −28.21% −28.14% −31.23% 1335% 239% C Class E Over- −0.74% −0.13% −0.16%  99% 101% −33.88% −35.35% −36.61% 1074% 232% all Class −0.23% −1.35% −0.09% 100% 101% −26.31% −26.11% −28.32% 1526% 242% D Class −0.30% −1.31% −0.89%  99% 100% −40.11% −41.99% −43.80%  737% 207% F (op- tional) VTM5.0 Over HM16.20 Y U V EncT DecT Class −34.92% −39.81% −43.03%  921% 228% A1 Class −39.04% −37.01% −30.74% 1005% 233% A2 Class −32.76% −37.86% −40.19% 1006% 223% B Class −27.86% −27.10% −31.11% 1309% 230% C Class E Over- −33.14% −35.21% −36.45% 1060% 228% all Class −26.07% −24.83% −28.17% 1492% 232% D Class −39.81% −40.67% −42.91%  725% 201% F (op- tional) Low delay B Main10 Delta Over VTM−5.0 Proposal Over HM16.20 Y U V EncT DecT Y U V EncT DecT Class A1 Class A2 Class −0.01% −4.66% −3.77% 100% 101% −25.95% −26.41% −27.21%  834% 206% B Class −0.04% −2.31% −1.44% 100%  99% −22.51% −21.31% −23.43% 1009% 201% C Class  0.10% −7.41% −6.86%  99% 101% −25.48% −28.30% −31.92%  440% 164% E Over-  0.01% −3.80% −2.98% 100% 100% −24.69% −25.18% −27.13%  757% 193% all Class −0.01% −4.10% −1.93% 101% 104% −21.24% −17.86% −19.42% 1104% 213% D Class −0.16% −1.67% −1.24%  99% 100% −36.47% −37.86% −39.29%  556% 168% F (op- tional) VTM5.0 Over HM16.20 Y U V EncT DecT Class A1 Class A2 Class −25.94% −21.75% −23.44%  815% 196% B Class −22.47% −18.99% −21.99%  985% 197% C Class −25.58% −23.94% −28.23%  433% 160% E Over- −24.69% −21.38% −24.15%  741% 187% all Class −21.22% −13.76% −17.49% 1069% 200% D Class −36.30% −36.19% −38.05%  544% 165% F (op- tional) Low delay P Main10 Delta Over VTM−5.0 Proposal Over HM16.20 Y U V EncT DecT Y U V EncT DecT Class A1 Class A2 Class −0.03% −4.60% −3.55%  99% 100% −30.44% −29.55% −30.15%  771% 211% B Class −0.12% −2.54% −1.42%  99% 100% −24.69% −22.33% −24.25%  936% 211% C Class  0.04% −3.84% −3.99% 100% 101% −29.13% −32.44% −36.22%  392% 171% E Over- −0.04% −3.72% −2.95%  99% 100% −28.19% −27.87% −29.70%  694% 200% all Class  0.04% −4.34% −2.93% 100% 101% −22.94% −19.34% −20.46% 1041% 208% D Class −0.16% −1.34% −1.38% 100% 100% −36.27% −37.56% −39.27%  579% 175% F (op- tional) VTM5.0 Over HM16.20 Y U V EncT DecT Class A1 Class A2 Class −30.41% −24.95% −26.60%  756% 206% B Class −24.57% −19.79% −22.83%  918% 206% C Class −29.16% −28.61% −32.22%  385% 166% E Over- −28.15% −24.15% −26.75%  681% 195% all Class −22.98% −15.00% −17.53% 1021% 201% D Class −36.11% −36.22% −37.88%  565% 171% F (op- tional)

Using of adjusted table provides coding gain of 0.74%/0.13%/0.17% (Y/Cb/Cr) in RA configuration over VTM5.0 and reported more balanced VTM5.0 coding gain over HM across luma and chroma components, −23.90%/−22.43%/−24.96%, −33.88%/−35.36%/−36.61%, −24.69%/−25.18%/−27.13% and −28.19%/−27.87%/−29.70% (Y/Cb/Cr) in AI, RA, LDB and LDP configuration, respectively.

As can be seen from tables above usage of new mapping function represented as look-up table as described in above embodiment provides more balanced coding gain between luma and chroma components. Another significant benefit is that for Random Access configuration it also provides substantial coding gain for both luma and chroma components, which is valuable since Random Access configuration is most used configuration in real application scenarios targeted to general purpose video coding for storing and broadcasting.

In a third embodiment the mapping function is defined as follows (also shown in FIG. 8 as lookup table 800):

QPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QPc = QPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 = QPi − 3

Enhanced Video Coding (EVC) is newly developing standard. The table below demonstrates effect of using mapping function according to the third embodiment in EVC Test Model v.2.0 (ETM2.0)

Random Access Main 10 Over HM16.16 Over ETM2.0 VAR1 Y U V EncT DecT Y U V EncT DecT Class A −26.61% −24.65% −24.23% 338% 171% −0.17% −6.72% −6.18%  99%  99% Class B −19.10% −21.80% −19.08% 398% 149%  0.06% −7.38% −7.43% 100%  99% Class E Overall −22.86% −23.22% −21.66% 367% 160% −0.05% −7.05% −6.80%  99%  99% Class S  −8.33% −17.12% −16.75% 429% 134% Class C −13.61% −15.86% −16.92% 521% 147% −0.02% −5.01% −4.59%  99%  99% Class D −12.00% −15.86% −16.06% 518% 145%  0.04% −6.00% −5.34% 100% 101%

As can be seen using of the mapping function according to the third embodiment provides average coding gain both for luma and chroma components and allows to make compression efficiency over HM balanced across luma and chroma components.

This mapping function according to the third embodiment is designed for coding tools set specific for EVC standard and provides balanced luma/chroma gain and average improvements in all three components.

To illustrate the usage of look-up table containing mapping function in EVC standard the following document is cited: ISO/IEC FDIS 23094-1 ISO/IEC JTC1/SC 29/WG 11 document w19229 “Information technology—General video coding—Part 1: Essential video coding”, FDIS stage.

7.4.3 Raw Byte Sequence Payloads, Trailing Bits and Byte Alignment Semantics

7.4.3.1 SPS RBSP Semantics

-   -   . . . .     -   chromaqp_table_present_flag equal to 1 specifies that user         defined chroma QP mapping tables ChromaQpTable are signalled.         chroma_qp_table_present_flag equal to 0 specifies that user         defined chroma QP mapping tables are not signalled and variable         ChromaQpTable are initialized as follows:         -   If ChromaArrayType is equal to 1, the following applies:             -   If sps_iqt_flag is equal to 0, the variables                 ChromaQpTable[m][qPi] with m being equal to 0 and 1, and                 qPi being in the range of −QpBdOffsetC to 57 are set                 equal to the value of Qp_(C) as specified in Table 5                 based on the index qPi.             -   Otherwise (sps_iqt_flag is equal to 1), the variables                 ChromaQpTable[m][qPi] with m equal to 0 and 1, and qPi                 being in the range of −QpBdOffsetC to 57 set equal to                 the value of Qp_(C) as specified in Table 6 based on the                 index qPi.         -   Otherwise, ChromaQpTable[m][qPi] with m being equal to 0 and             1, and qPi being in the range of −QpBdOffsetC to 57 are set             equal to the value of qPi.

TABLE 5 Specification of Qp_(c) as a function of qPi (sps_iqt_flag = = 0) qPi <30 30 31 32 33 34 35 36 Qp_(c) = qPi 29 29 29 30 31 32 32 qPi 44 45 46 47 48 49 50 51 Qp_(c) 36 36 37 37 37 38 38 39 qPi 37 38 39 40 41 42 43 Qp_(c) 33 33 34 34 35 35 36 qPi 52 53 54 55 56 57 58 Qp_(c) 39 40 40 40 41 41 41

TABLE 6 Specification of Qpc as a function of qPi (sps_iqt_flag = = 1) qPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 Qp_(c) = qPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 = qPi − 3 When not present, the value of chroma_qp_table_present_flag is inferred to be equal to 0. . . . 8.7 Scaling, Transformation and Array Construction Process 8.7.1 Derivation Process for Quantization Parameters

Input to this process is a luma location (xCb, yCb) specifying the top-left sample of the current luma coding block relative to the top-left luma sample of the current picture.

In this process, the variable Qp_(Y), the luma quantization parameter Qp′_(Y), and the chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) are derived.

The variable Qp_(Y) is derived as follows: Qp _(Y)=(Qp _(Y_PREV) +CuQpDelta[xCb][yCb]+52)%52  (1042) where Qp_(Y_PREV) is the luma quantization parameter, Qp_(Y) of the previous coding unit in decoding order in the current tile. For the first coding unit in the tile, Qp_(Y_PREV) is initially set equal to slice_qp at the start of each tile. The luma quantization parameter Qp′_(Y) is derived as follows: Qp′ _(Y) =Qp _(Y) +QpBdOffset_(Y)  (1043) When ChromaArrayType is not equal to 0, the following applies:

The variables Qp_(Cb) and Qp_(Cr) are derived as follows: qPi _(Cb)=Clip3(−QpBdOffset_(C),57,Qp _(Y)+slice_cb_qp_offset)  (1044) qPi _(Cr)=Clip3(−QpBdOffset_(C),57,Qp _(Y)+slice_cr_qp_offset)  (1045) Qp _(Cb)=ChromaQpTable[0][qPi _(Cb)]  (1046) Qp _(Cr)=ChromaQpTable[1][qPi _(Cr)]  (1047)

The chroma quantization parameters for the Cb and Cr components, Qp′_(Cb) and Qp′_(Cr), are derived as follows: Qp′ _(Cb) =Qp _(Cb) +QpBdOffset_(C)  (1048) Qp′ _(Cr) =Qp _(Cr)+QpBdOffset_(C)  (1049) 8.7.2 Scaling and Transformation Process

Inputs to this process are:

-   -   a luma location (xTbY, yTbY) specifying the top-left sample of         the current luma transform block relative to the top-left luma         sample of the current picture,     -   a variable cIdx specifying the colour component of the current         block,     -   a variable nTbW specifying the width of the current block, and     -   a variable nTbH specifying the height of the current block.         Output of this process is the (nTbW)×(nTbH) array of residual         samples r with elements r[x][y].         The quantization parameter qP is derived as follows:

If cIdx is equal to 0, the following applies. qP=Qp′ _(Y)  (1050)

Otherwise, if cIdx is equal to 1, the following applies. qP=Qp′ _(Cb)  (1051)

Otherwise (cIdx is equal to 2), the following applies. qP=Qp′ _(Cr)  (1052)

The (nTbW)×(nTbH) array of residual samples r is derived as follows:

-   -   1) The scaling process for transform coefficients as specified         in subclause 8.7.3 is invoked with the transform block location         (xTbY, yTbY), the transform block width nTbW and the transform         block height nTbH, the colour component variable cIdx, and the         quantization parameter qP as inputs, and the output is an         (nTbW)×(nTbH) array of scaled transform coefficients d.     -   2) The transformation process for scaled transform coefficients         as specified in subclause 8.7.4 is invoked with the transform         block location (xTbY, yTbY), the transform block width nTbW and         the transform block height nTbH, the colour component variable         cIdx, and the (nTbW)×(nTbH) array of scaled transform         coefficients d as inputs, and the output is an (nTbW)×(nTbH)         array of residual samples r.     -   3) The variable bdShift is derived as follows:         -   If sps_iqt_flag is equal to 0, the following applies:             bdShift=((cIdx==0)?20−BitDepth_(Y):20−BitDepth_(C))+7  (1053)         -   Otherwise (sps_iqt_flag is equal to 1), the following             applies:             bdShift=(cIdx==0)?20−BitDepth_(Y):20−BitDepth_(C)  (1054)     -   4) The residual sample values r[x][y] with x=0 . . . nTbW−1, y=0         . . . nTbH−1 are modified as follows:         r[x][y]=(r[x][y]+(1<<(bdShift−1)))>>bdShift  (1055)         8.7.3 Scaling Process for Transform Coefficients         Inputs to this process are:     -   a luma location (xTbY, yTbY) specifying the top-left sample of         the current luma transform block relative to the top-left luma         sample of the current picture,     -   a variable nTbW specifying the width of the current block,     -   a variable nTbH specifying the height of the current block,     -   a variable cIdx specifying the colour component of the current         block, and     -   a variable qP specifying the quantization parameter.         Output of this process is the (nTbW)×(nTbH) array d of scaled         transform coefficients with elements d[x][y].         The variable bdShift is derived as follows:

If cIdx is equal to 0, the following applies: bdShift=BitDepth_(Y)+(((Log 2(nTbW)+Log 2(nTbH))& 1)*8+(Log 2(nTbW)+Log 2(nTbH))/2)−5  (1056)

Otherwise, the following applies: bdShift=BitDepth_(C)+(((Log 2(nTbW)+Log 2(nTbH))& 1)*8+(Log 2(nTbW)+Log 2(nTbH))/2)−5  (1057) The variable rectNorm is derived as follows: rectNorm=((Log 2(nTbW)+Log 2(nTbH))&1)==1?181:1  (1058) The list levelScale[ ] is specified as follows:

-   -   If sps_iqt_flag is equal to 0, levelScale[k]={40, 45, 51, 57,         64, 71} with k=0 . . . 5.     -   Otherwise (sps_iqt_flag is equal to 1), levelScale[k]={40, 45,         51, 57, 64, 72} with k=0 . . . 5.         For the derivation of the scaled transform coefficients d[x][y]         with x=0 . . . nTbW−1, y=0 . . . nTbH−1, the following applies:     -   The scaled transform coefficient d[x][y] is derived as follows:         d[x][y]=Clip3(−32768,32767,((TransCoeffLevel[xTbY][yTbY][cIdx][x][y]*levelScale[qP         %6]<<(qP/6))*rectNorm+(1<<(bdShift−1)))>>bdShift)  (1059)

FIG. 9 is a block diagram illustrating a decoder 930 and an encoder 920, comprising one or more processors 901; and a non-transitory computer-readable storage medium 902 coupled to the processors 901 and storing programming for execution by the one or more processors 901, wherein the programming, when executed by the one or more processors 901, configures the decoder 930 or encoder 920 to carry out the method of the present disclosure.

FIG. 10 is a block diagram illustrating a decoder 1030 for obtaining a quantization parameter (QP) for a chrominance component based on a QP for a luminance component. The decoder 1030 comprises a first obtaining unit 1031 configured to obtain a luminance QP; a second obtaining unit 1032 configured to obtain a QP index (QPi) based at least in a part on the luminance QP; and a third obtaining unit 1033 configured to obtain a chrominance QP (QPc) based on the QP index by using a preset lookup table, wherein the maximum difference between the QP index and the chrominance QP is equal to 3. The decoder may further comprise an adjusting unit (1034) for adjusting the obtained chrominance QP by adding an offset value and may comprise a clipping unit (1035) for clipping the QP index into a preset consecutive value range. Adding an offset value to the QP value and clipping the QP index has been described above.

The present disclosure provides the following further embodiments 1 to 8:

Further Embodiment 1. A method of obtaining quantization parameter (QP) for chrominance components based on QP for luminance component, wherein the method is performed by a decoder or encoder, comprising:

-   -   obtaining a luminance QP;     -   obtaining a QP index (QPi) based at least in a part on the         luminance QP;     -   obtaining a chrominance QP (QPc) based on the QP index by using         a mapping function or using a lookup table wherein the QP index         and a chrominance QP associated to the QP index is defined, and         the maximum difference between the QP index and the chrominance         QP is K, K<6.

Further Embodiment 2. The method of further embodiment 1, wherein the mapping function is a monotonically increasing (non-decreasing) function.

Further Embodiment 3. The method of further embodiment 2, wherein the K is an integer and equal to 3.

Further Embodiment 4. The method of further embodiment 3, wherein the lookup table is as follows:

QPi <35 35 36 37 38 39 40 41 42 43 >43 QP_(c) = QPi 34 35 36 37 37 38 39 40 40 = QPi − 3

Further Embodiment 5. The method of further embodiment 3, wherein the lookup table is as follows::

QPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QPc = QPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 = QPi − 3

Further Embodiment 6. An encoder (20) or a decoder (30) comprising processing circuitry for carrying out the method according to any one of further embodiments 1 to 5

Further Embodiment 7. A computer program product comprising a program code for performing the method according to any one of further embodiments 1 to 5.

Further Embodiment 8. A decoder or encoder, comprising:

-   -   one or more processors; and     -   a non-transitory computer-readable storage medium coupled to the         processors and storing programming for execution by the         processors, wherein the programming, when executed by the         processors, configures the decoder to carry out the method         according to any one of further embodiments 1 to 5.         Mathematical Operators

The mathematical operators used in this application are similar to those used in the C programming language. However, the results of integer division and arithmetic shift operations are defined more precisely, and additional operations are defined, such as exponentiation and real-valued division. Numbering and counting conventions generally begin from 0, e.g., “the first” is equivalent to the 0-th, “the second” is equivalent to the 1-th, etc.

Arithmetic Operators

The following arithmetic operators are defined as follows:

-   -   + Addition     -   − Subtraction (as a two-argument operator) or negation (as a         unary prefix operator)     -   * Multiplication, including matrix multiplication     -   x^(y) Exponentiation. Specifies x to the power of y. In other         contexts, such notation is used for superscripting not intended         for interpretation as exponentiation.     -   / Integer division with truncation of the result toward zero.         For example, 7/4 and −7/−4 are truncated to 1 and −7/4 and 7/−4         are truncated to −1.     -   ÷ Used to denote division in mathematical equations where no         truncation or rounding is intended.

$\frac{x}{y}$

-   -   Used to denote division in mathematical equations where no         truncation or rounding is intended.

$\sum\limits_{i = x}^{y}{f(i)}$

-   -   The summation of f(i) with i taking all integer values from x up         to and including y. Modulus. Remainder of x divided by y,         defined only for integers x and y with x>=0 x % y and y>0.         Logical Operators

The following logical operators are defined as follows:

-   -   x && y Boolean logical “and” of x and y     -   x∥y Boolean logical “or” of x and y     -   ! Boolean logical “not”     -   x?y: z If x is TRUE or not equal to 0, evaluates to the value of         y; otherwise, evaluates to the value of z.         Relational Operators

The following relational operators are defined as follows:

-   -   < Greater than     -   >= Greater than or equal to     -   > Less than     -   <= Less than or equal to     -   == Equal to     -   != Not equal to

When a relational operator is applied to a syntax element or variable that has been assigned the value “na” (not applicable), the value “na” is treated as a distinct value for the syntax element or variable. The value “na” is considered not to be equal to any other value.

Bit-Wise Operators

The following bit-wise operators are defined as follows:

-   -   & Bit-wise “and”. When operating on integer arguments, operates         on a two's complement representation of the integer value. When         operating on a binary argument that contains fewer bits than         another argument, the shorter argument is extended by adding         more significant bits equal to 0.     -   | Bit-wise “or”. When operating on integer arguments, operates         on a two's complement representation of the integer value. When         operating on a binary argument that contains fewer bits than         another argument, the shorter argument is extended by adding         more significant bits equal to 0.     -   {circumflex over ( )} Bit-wise “exclusive or”. When operating on         integer arguments, operates on a two's complement representation         of the integer value. When operating on a binary argument that         contains fewer bits than another argument, the shorter argument         is extended by adding more significant bits equal to 0.     -   x>>y Arithmetic right shift of a two's complement integer         representation of x by y binary digits. This function is defined         only for non-negative integer values of y. Bits shifted into the         most significant bits (MSBs) as a result of the right shift have         a value equal to the MSB of x prior to the shift operation.     -   x<<y Arithmetic left shift of a two's complement integer         representation of x by y binary digits. This function is defined         only for non-negative integer values of y. Bits shifted into the         least significant bits (LSBs) as a result of the left shift have         a value equal to 0.         Assignment Operators

The following arithmetic operators are defined as follows:

-   -   = Assignment operator     -   ++ Increment, i.e., x++ is equivalent to x=x+1; when used in an         array index, evaluates to the value of the variable prior to the         increment operation.     -   −− Decrement, i.e., x−− is equivalent to x=x−1; when used in an         array index, evaluates to the value of the variable prior to the         decrement operation.     -   += Increment by amount specified, i.e., x+=3 is equivalent to         x=x+3, and x+=(−3) is equivalent to x=x+(−3).     -   −= Decrement by amount specified, i.e., x−=3 is equivalent to         x=x−3, and x−=(−3) is equivalent to x=x−(−3).         Range Notation

The following notation is used to specify a range of values:

-   -   x=y . . . z x takes on integer values starting from y to z,         inclusive, with x, y, and z being integer numbers and z being         greater than y.         Mathematical Functions

The following mathematical functions are defined:

$\begin{matrix} {{A\;{{bs}(x)}} = \left\{ \begin{matrix} {x;{x>=0}} \\ {{- x};{x < 0}} \end{matrix} \right.} & \; \end{matrix}$

-   -   A sin(x) the trigonometric inverse sine function, operating on         an argument x that is in the range of −1.0 to 1.0, inclusive,         with an output value in the range of −π÷2 to π÷2, inclusive, in         units of radians     -   A tan(x) the trigonometric inverse tangent function, operating         on an argument x, with an output value in the range of −π÷2 to         π÷2, inclusive, in units of radians

${A\tan 2\left( {y,x} \right)} = \left\{ \begin{matrix} {{A\;{\tan\ \left( \frac{y}{x} \right)}};\ {x > 0}} \\ {{{{A\tan}\ \left( \frac{y}{x} \right)} + \pi};\ {{{{{{x < 0}\ \&}\&}\ y} >} = 0}} \\ {{{A\;{\tan\ \left( \frac{y}{x} \right)}} - \pi};\ {{{{{x < 0}\ \&}\&}\ y} < 0}} \\ {{+ \frac{\pi}{2}}\ ;{x=={{{{{0\ \&}\&}\ y} >} = 0}}} \\ {{- \ \frac{\pi}{2}}\ ;{otherwise}} \end{matrix} \right.$

-   -   Ceil(x) the smallest integer greater than or equal to x.         Clip1_(Y)(x)=Clip3(0,(1<<BitDepth_(Y))−1,x)         Clip1_(C)(x)=Clip3(0,(1<<BitDepth_(C))−1,x)

${{Clip}\mspace{11mu} 3\left( {x,y,z} \right)} = \left\{ \begin{matrix} {x;\ {z < x}} \\ {y;\ {z > y}} \\ {z\ ;{otherwise}} \end{matrix} \right.$

-   -   Cos(x) the trigonometric cosine function operating on an         argument x in units of radians     -   Floor(x) the largest integer less than or equal to x.

${{Get}\;{{CurrMsb}\left( {a,b,c,d} \right)}} = \left\{ \begin{matrix} {{c + d}\ ;{{b - a}\ >={d/2}}} \\ {{c - d};\ {{a - b}\  > {d/2}}} \\ {c;\mspace{14mu}{otherwise}} \end{matrix} \right.$

-   -   Ln(x) the natural logarithm of x (the base-e logarithm, where e         is the natural logarithm base constant 2.718 281 828 . . . ).     -   Log 2(x) the base-2 logarithm of x.     -   Log 10(x) the base-10 logarithm of x.

${{Min}\left( {x,y} \right)} = \left\{ {{\begin{matrix} {x;{x<=y}} \\ {y;{x > y}} \end{matrix}{{Max}\left( {x,y} \right)}} = \left\{ \begin{matrix} {x;{x>=y}} \\ {y;{x < y}} \end{matrix} \right.} \right.$

-   -   Round(x)=Sign(x)*Floor(Abs(x)+0.5)

$\begin{matrix} {{{Sign}\mspace{11mu}(x)} = \left\{ \begin{matrix} {1;\ {x > 0}} \\ {0;\ {x = {= 0}}} \\ {{- 1};\ {x < 0}} \end{matrix} \right.} & \; \end{matrix}$

-   -   Sin(x) the trigonometric sine function operating on an argument         x in units of radians     -   Sqrt(x)=√x     -   Swap(x, y)=(y, x)     -   Tan(x) the trigonometric tangent function operating on an         argument x in units of radians         Order of Operation Precedence

When an order of precedence in an expression is not indicated explicitly by use of parentheses, the following rules apply:

-   -   Operations of a higher precedence are evaluated before any         operation of a lower precedence.     -   Operations of the same precedence are evaluated sequentially         from left to right.

The table below specifies the precedence of operations from highest to lowest; a higher position in the table indicates a higher precedence.

For those operators that are also used in the C programming language, the order of precedence used in this Specification is the same as used in the C programming language.

Table: Operation Precedence from Highest (at Top of Table) to Lowest (at Bottom of Table)

operations (with operands x, y, and z) ″x+ +″, ″x− −″ ″!x″, ″−x″ (as a unary prefix operator) x^(y) ″x * y″, ″x/y″, ″x ÷ y″, ″ x/y″, ″x % y″ ${\;^{''}x + {y^{''}{,\mspace{14mu}}^{''}x} - {y^{''}\left( {{as}\mspace{14mu} a\mspace{14mu}{two}\text{-}{argument}\mspace{14mu}{operator}} \right)}},^{\mspace{14mu}''}{\sum\limits_{i = x}^{y}{f(i)}^{''}}$ ″x << y″, ″x >> y″ ″x < y″, ″x <= y″, ″x > y″, ″x >= y″ ″x = = y″, ″x ! = y″ ″x & y″ ″x | y″ ″x && y″ ″x ∥ y″ ″x ? y : z″ ″x . . . y″ ″x = y″, ″x + = y″, ″x − = y″ Text Description of Logical Operations

In the text, a statement of logical operations as would be described mathematically in the following form:

 if( condition 0)   statement 0  else if( condition 1)   statement 1  ...  else /* informative remark on remaining condition */   statement n may be described in the following manner:  ... as follows / ... the following applies:    If condition 0, statement 0    Otherwise, if condition 1, statement 1    ...    Otherwise (informative remark on remaining    condition), statement n

Each “If . . . Otherwise, if . . . Otherwise, . . . ” statement in the text is introduced with “ . . . as follows” or “ . . . the following applies” immediately followed by “If . . . ”. The last condition of the “If . . . Otherwise, if . . . Otherwise, . . . ” is always an “Otherwise, . . . ”. Interleaved “If . . . Otherwise, if . . . Otherwise, . . . ” statements can be identified by matching “ . . . as follows” or “ . . . the following applies” with the ending “Otherwise, . . . ”.

In the text, a statement of logical operations as would be described mathematically in the following form:

 if( condition 0a && condition 0b)   statement 0  else if( condition 1a | | condition 1b)   statement 1  ...  else   statement n may be described in the following manner:  ... as follows / ... the following applies:    If all of the following conditions are true, statement 0:     condition 0a     condition 0b    Otherwise, if one or more of the following conditions    are true, statement 1:     condition 1a     condition 1b    ...    Otherwise, statement n

In the text, a statement of logical operations as would be described mathematically in the following form:

 if( condition 0)    statement 0  if( condition 1)    statement 1 may be described in the following manner:   When condition 0, statement 0   When condition 1, statement 1

Although embodiments of the invention have been primarily described based on video coding, it should be noted that embodiments of the coding system 10, encoder 20 and decoder 30 (and correspondingly the system 10) and the other embodiments described herein may also be configured for still picture processing or coding, i.e. the processing or coding of an individual picture independent of any preceding or consecutive picture as in video coding. In general only inter-prediction units 244 (encoder) and 344 (decoder) may not be available in case the picture processing coding is limited to a single picture 17. All other functionalities (also referred to as tools or technologies) of the video encoder 20 and video decoder 30 may equally be used for still picture processing, e.g. residual calculation 204/304, transform 206, quantization 208, inverse quantization 210/310, (inverse) transform 212/312, partitioning 262/362, intra-prediction 254/354, and/or loop filtering 220, 320, and entropy coding 270 and entropy decoding 304.

Embodiments, e.g. of the encoder 20 and the decoder 30, and functions described herein, e.g. with reference to the encoder 20 and the decoder 30, may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on a computer-readable medium or transmitted over communication media as one or more instructions or code and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.

By way of example, and not limiting, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 

The invention claimed is:
 1. A method of obtaining quantization parameters (QPs) for chrominance components based on QPs for luminance components, wherein the method is performed by a decoder or encoder, comprising: obtaining a luminance QP for a luma component of a coding unit; obtaining a QP index (QPi) based at least in a part on the luminance QP; obtaining a chrominance QP (QPc) for a chroma component of the coding unit based on the QP index using a preset lookup table, wherein the maximum difference between the QP index and the QPc is equal to
 3. 2. The method of claim 1, wherein the preset lookup table is defined by a monotonically increasing mapping function.
 3. The method of any of claim 2, wherein the mapping function is QPc=QPi for QPi<QPi1; QPc=QPi−1 for QPi1≤QPi<QPi2; QPc=QPi−2 for QPi2≤QPi<QPi3; QPc=QPi−3 for QPi≥QPi3, wherein QPi1, QPi2 and QPi3 are integers with QPi1<QPi2<QPi3.
 4. The method of claim 3, wherein QPi1=35 or 30, QPi2=39, QPi3=43.
 5. The method of claim 1, comprising: adjusting the obtained QPc by adding an offset value.
 6. The method of claim 1, wherein, the QPi is in a preset consecutive value range.
 7. The method of claim 1, wherein the lookup table is as follows: QPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QPc = QPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 = QPi −
 3.


8. The method of claim 1, wherein the lookup table is as follows: QPi <35 35 36 37 38 39 40 41 42 43 >43 QP_(c) = QPi 34 35 36 37 37 38 39 40 40 = QPi −
 3.


9. The method of claim 1, wherein the same lookup table is applied for each chrominance component Cb and Cr or jointly coded Cb-Cr.
 10. The method of claim 1, further comprising: receiving a bitstream; and parsing the bitstream to derive the luminance QP and syntax elements specifying the lookup table.
 11. A decoder or encoder, comprising: one or more processors; and a non-transitory computer-readable storage medium coupled to the one or more processors and storing programming for execution by the one or more processors, wherein the programming, when executed by the one or more processors, causes the decoder or encoder to: obtain a luminance QP for a luma component of a coding unit; obtain a QP index (QPi) based at least in a part on the luminance QP; obtain a chrominance QP (QPc) for a chroma component of the coding unit based on the QP index by using a preset lookup table, wherein the maximum difference between the QP index and the chrominance QP is equal to
 3. 12. The decoder or encoder of claim 11, wherein the preset lookup table is defined by a monotonically increasing mapping function.
 13. The decoder or encoder of any of claim 12, wherein the mapping function is QPc=QPi for QPi<QPi1; QPc=QPi−1 for QPi1≤QPi<QPi2; QPc=QPi−2 for QPi2≤QPi<QPi3; QPc=QPi−3 for QPi≥QPi3, wherein QPi1, QPi2 and QPi3 are integers with QPi1<QPi2<QPi3.
 14. The decoder or encoder of claim 13, wherein QPi1=35 or 30, QPi2=39, QPi3=43.
 15. The decoder or encoder of claim 11, wherein the programming, when executed by the one or more processors, further causes the decoder or encoder to: adjust the obtained chrominance QP by adding an offset value.
 16. The decoder or encoder of claim 11, wherein, the QP index is in a preset consecutive value range.
 17. The decoder or encoder of claim 11, wherein the lookup table is as follows: QPi <30 30 31 32 33 34 35 36 37 38 39 40 41 42 43 >43 QPc = QPi 29 30 31 32 33 34 35 36 37 37 38 39 40 40 = QPi −
 3.


18. The decoder or encoder of claim 11, wherein the lookup table is as follows: QPi <35 35 36 37 38 39 40 41 42 43 >43 QP_(c) = QPi 34 35 36 37 37 38 39 40 40 = QPi −
 3.


19. The decoder or encoder of claim 11, wherein the same lookup table is applied for each chrominance component Cb and Cr or jointly coded Cb-Cr; or wherein different lookup tables are applied for chrominance components Cb and Cr or jointly coded Cb-Cr.
 20. The decoder or encoder of claim 11, wherein the programming, when executed by the one or more processors, further causes the decoder or encoder to: receive a bitstream; and parse the bitstream to derive the luminance QP and syntax elements specifying the lookup table. 